If the collective mind contains memories of people, well... It contains what people remember, not the factual truth. This poses multiple problems:

• Conflicting memories, which may either be mutually destroyed upon merger, or lead to great levels of confusion;




• Poor attention. You have found the memories of the sole witness of whatever, but they didn't see the murder because they were playing some My Little Pony game on their cell phone.




• Fragmented memories. It is quite common for people who have undergone post traumatic stress or child abuse to have a kind of dissociation called fragmentation. This screws up the way the person collects, maintains and recalls memories. I think murder victims would have poor memories of their last moments.




• False memories, planted on purpose to confuse the oversoul, either by one's own self and volition or forcefully.




• The oversoul will have waaaaay too many stoned memories. Seeing history through the eyes of a junkie may be interesting, but not very useful for fact finding.




• Storage. The oversoul has more capacity than you. It may be that in order to get some info from it, you have to forget an equal amount of info just to make space.




• Emotional content. If you experience someone else's memories, you may feel what they felt. You will need psychological help if you go through a rape victim's memory.

Assassin's Creed has a similar concept, and its lore added two additional problems for people going through the memories of others:

• Poor search indexing. You can't just go to that single precious moment which is all that you want. You have to spend 20-odd hours playing previous memories until you reach what you want.




• Bleeding effect^TM: spend too much time visiting memories and you start hallucinating pieces of those memories even when unplugged from their source.

Read the histories of the oracles and you'll see they have certain quirks. Consider Cassandra, her curse was that her predictions were always accurate but that nobody would ever believe her. Other oracles were known for giving riddles or predictions that couldn't be interpreted until it was too late.

Your Oracles can see clearly into the past, but not so clearly into the present. That means that while they know exactly what happened, they're not very good at communicating it to others. In the simplest case they're talking to someone who was asking questions last week. In the worst case no two consecutive words are from the same conversation.

Riddles, confusion, and disconnection in time. The oracles know everything, they just can't tell you about it.

The next step is to find a balance.

An oracle at the pinnacle of their profession will be able to speak to a single person about an event with no other witnesses, say the victim of a murder, but they're also going to be the worst case of being unable to communicate with the present. A new oracle is still going to be relatively well grounded in time, but only be able to find out about events with loud voices in the past, great wars and civilisations with tens of thousands of witnesses. Their speech may only be light riddles, but the fine detail can never be seen.

It's very simple for two reasons.

• Reason 1: It is a collective memory of DEAD people. You can't remember the juicy details about your hot neighbour or the bank account numbers of a rich celebrity if they didn't die. You could get some information on the enemy troops after people have died, but you can't be sure how accurate that information is depending on who died and what they knew. "depending on who died" means that if you kill a captured officer you can be more certain about the information he'll provide.




• Reason 2: It's a COLLECTIVE memory. You ask information about some moment and place in history, everyone who feels they know something about it will respond. Steering through this mess is difficult for the user. This also means you get more clear info about things that more people know about. A murder on an important leader on national TV will be much clearer than a random death in a back alley, the back alley death might be so small that it's simply drowned out and will be mixed with similar deaths in the region and time period, making the information tainted and hard to understand.

First, all of human history is...BIG. So what you'd want is specialists in particular areas of history.

Here's some fixes:

• 
  Time. You've got history, but immediate history is more difficult and confused to access. If you get anything at all, it's fractured. As time goes by, it gains clarity. 100 years ago is clearer than 1 year ago as the timelines of all the dead folk have had time to become part of the whole in an integrated and accessible way. For a god-like consciousness, several decades or a century isn't a big deal, and really, it might be a safety feature for just the kind of thing you are talking about.


• 
  Too Many Voices. The biggest problem here is organization and indexing. How can everyone possibly bring up the information needed? How can you hear one voice when there are millions? What this might mean is that bringing up info is difficult.


• 
  Lies, delusions and misremberances. Even if you can get the dead dude you're looking for, there's no guarantee that his perceptions are accurate. You call it a "perfect recording" but if humans are involved, dead or alive, there is really no such thing if you also have "Thoughts, emotions, ideas." Take a moment to research eyewitness testimony. Memory is a funny thing, and it's NOT perfect. I know your idea is perfect accuracy. But more than one thing can be true at a time. Consider the story of Logain from Dragon Age--some remember him as a hero who saved his troops from an overwhelming enemy and their country from a King who was beginning to collude with an old enemy, others consider him a traitor who left his king on the field. BOTH of those things could be and are true. It really depends on who you listen to.

You're talking about ONE linear, TRUE version of history, but I contend that there really is no such thing. Every event ever has depended on interpretation, on the perception of those involved.

I really do believe that it would be FAR, FAR more interesting and accurate if it was more confusing and harder to access. And what's true...that's not one thing. It never has been.

"This way, there can be one, linear, version of historical events that dont depend on interpretation, "

RL Example: In WW2, During Battle of Britain both sides were convinced that RAF was on verge of collapse. This belief shaped strategical decisions - RAF even started saving aircrafts, in order to still have some reserve, which made Germans think that not much is left. The only problem... recent studies show that both sides incorrectly estimated other side production rate and in the darkest hour Allies were decisively winning aerial war of attrition.

This information may be hard to get from a collective consciousness, but a dispassionate quartermaster (or accountant) may be a bit more useful. ;)

You don't want to make it a game breaking? Except already raised issues like false memories (including hallucinations), emotions... Why the process should be immediate? I mean if a soul needs a few decades to truly merge, then it does not matter for ancient history but makes it mostly useless from military perspective.

There may be even some interesting mechanism that for example sinner, individualist or very stron personalities would simply need a much more time to merge.

EDIT: Events equivalent to... evolving on savanna? Almost getting extinct 70k years ago? Leaving Africa? Ice age cycles? Slaughtering all megafauna on the way? Having sex with other hominids like Neanderthals and Denisovans? Domesticating plants and animals? Going in huge migrations throughout continents? (recent DNA test show that Europeans are mixture of local population + Middle Eastern + Siberian)

It can be quite interesting as history would start at dawn of mankind and not at invention of writing.

/I need to limit this system to the big moves and shakes of history./

Signal strength increases with number of participants.

Imagine flying along at 10,000 feet. You cannot see a man lighting his cigarette. But you can see 5000 concert goers lifting their lighters for a torch song.

So too events. Participant number increases signal strength. If you want to see what OJ Simpson did on a given night, you have only the memories of the three individuals present as a source; too weak. If you want to see SuperBowl 44, you have the memories of thousands of attendees and millions who watched on TV. The more participants there are, the stronger your signal. You can set a number of participants necessary to make any perceptible signal for the oracles, and near that minimum number signal is still very weak.

How can I prevent an oracle who can see into the past from knowing
everything that has happened?

No-one has the big picture.

_{At least - not for long.}

Each death would represent only a small incremental increase in the God's information resource. This resembles a Clandestine cell system, such as used in spy networks. Orders would get passed down through ever shifting cut-outs, it would all consist of disjointed and sometimes contradictory information.

• It would take a great deal of analysis by the Oracles to find underlying paterns of government or military strategy - and even then false trails, double and tripple bluffs would muddy the waters.




• Leadership. Just as at the bottom of the hierarchy, the leaders would be surrounded by mutual anonymity - masks, mystery and would use secret passwords, handshakes and the signs and trappings of ritual to identify themselves and their position and role.




• Every time a leader or trusted adviser died, a randomisation strategy would need to be implemented, a replacement leader by lottery, a change of course in strategies - a random shifting of resources to again muddy the waters and make themselves and their plans safely obscured.




• Attempted infiltration of enemy cells, and climbing in their hierarchies would be something you'd constantly need to contend with and be attempting on your enemy.

Suitable citizens would be segregated depending on ability.

Intensive training would be given in:

• Oraclular studies, strategy and the history and practice of clandestine networks and disinformation systems, insurgency and counter insurgency - these would become the future leaders, academic advisors and teachers, the most trusted. They would be afforded luxury and a harem to share - but no personal attachments which could be used as leverage against them.




• Mathematics of codes and cyphers.




• The enemy's culture and language.




• Intelligence gathering techinques.




• Subversive-insurgency to infiltrate and disrupt enemy networks.




• Assasination. They would be threatened with hell-fire and promised rewards in heaven or rewards for their families on Earth to commit Lone-Wolf attacks on known enemy targets.

All in all, it's a recipe for scandal, intrigue and a pretty unpleasant and insecure workplace IMHO.

Location

A simple and interesting way to limit it is to do so by location. 'God' is spread out over the world. The memories it contains are tied to the area where they occurred. For an Oracle to read them, the Oracle must be fairly close by.

Want to have an Oracle solve a murder? She has to go to the crime scene.

Need to do a study of ancient Greece? You won't be doing it from a hotel in Paramus.

Pick out the details of your enemy's plans in the war? Than you allied Oracle will have to get into an enemy base, or even better, their headquarters. And even that won't help if everyone who was present and has those memories is still alive.

Concentration of memories determines clarity, accessibility, and importance.
Time spent in accessed memories moves slower.

If you look at WW2, there are your oracles who could study it all their lives and have meaningful things to discern and discuss with fellow oracles. The pool of memories would be that immense. Your detective would have to search for vaguer trails acting in solitude, or with a team. He does not have historians from across the world curious about this cataclysmic event in human history. WW2 would. We could reach your mentioned objectivity after a process of historical research using this unique avenue. However, it would take time and many capable minds, not one of which could be an island.

Your other valve for how potent the power is would be its cost. Similar to other magic systems, as this system may be considered, the cost is a great place to balance its strength and usefulness. When you are roaming through memories, how long does it take to get there? Can your brain maneuver through the collective conciousness with enough detachment to do so efficiently, or does your reaction to everything you see along the way slow you? Perhaps processing the information left by the dead is a draining, costly thing.

Even your modern spy would take too long searching memories if their brain must process each thing in a substantial way. Then there is that issue of finding something which is so fresh to the world. It has not a large enough trail to be easily accessed or navigated, further exponentially reducing your efficiently at gleaming useful information. The longer you dig, the vaguer the trail, and the more you have to process to keep going further. One man can only process so much information in a certain frame of time.

Background noise.

I need to limit this system to the big moves and shakes of history.
Events equivalent to WW2, rise and fall of Rome, cultures of
Mesopotamia, etc. The big and important moments that shaped the world
on the grand scheme, rather than the lives of individuals. This way,
there can be one, linear, version of historical events that dont
depend on interpretation, but leave the lives of people out. How can I
make this happen?

The data is there, but oracles have trouble exactly locating it. Because the Memory is so large that it's impossible to quickly zoom in any one point. And you cannot stay connected for very long, or your brain will seize.

So you can only zero on large events, something that left a large impression on the Memory. Once there, you can concentrate on smaller details; but you can't "remember" who killed Joe Q. Average, because the event didn't make a large enough impression. Even if it did, you'd be sore pressed in zeroing onto the memories of the actual murderer; and if he was the only one to know that he was indeed the murderer, connecting with any other memory will not help you.

Oracles can only communicate clearly with specific people.

Let’s imagine an Oracle A. If Oracle A has a specific trait 1, then A might be “spiritually closer” (or some other handwavium) to people who have this specific trait 1. Likewise, if they have a specific trait 2, then they’ll communicate best with people who have 2 (and 1).

This way, given enough traits or rare enough ones, you’ll cut down the number of people Oracle A can communicate with massively. So for a small event such as a murder where only a few people were involved, it’s unlikely to the point of impossibility that A will be able to recover information; however, for a massive event, A will have many witnesses they can draw from.

And who knows? Maybe you just have to find an Oracle B to get more information.

A side effect of this is that ancient history will be weaker, as less people were alive then. This could be either positive or negative.

You could make it take a really, really long time to find more obscure things. Everything that ever happened is a lot of data to sift through, so that should be reasonable enough. Like the internet, perhaps all of the major events could be well-indexed and easy to find. But the more specific and obscure things that an Oracle might want to know may be much harder to find. If the web page you're looking wasn't indexed by Google, it'd take a much longer time to find it, wouldn't you say? So as a limitation, just make it take an inconveniently long time to find things that are not the big moves and shakes of history. (This has also been brought up in this answer and this answer.)

Actually, this idea comes up in the manga ib: Instant Bullet by Akasaka Aka, and I think it presents the concept of the search duration limitation rather well (read right-to-left):

Even though we witness events, we don't always know what exactly is happening. The simplest example of this is close up magic.
So even though an Oracle may see something, they may miss what is actually happening.

Secondly, memories are a perspective. A person's memory is what they think they have seen.

Lastly, people don't remember things as they happened even though they may have witnessed them and understood them thoroughly at the moment they occurred.

Maybe these flaws can make prevent the oracles from seeing everything and limit their powers in some sense

I think there's actually an even simpler answer: context.

Even if the oracle can see exactly what the dead person saw, thought and felt, that doesn't mean that they understand it perfectly.

The most obvious example of this is language: I won't gain any insight into the causes of the Second World War from watching Hitler's speeches unless I can speak German - and that's a language which millions of people still speak today. What if I'm observing someone who spoke North Picene?

Context is important for everything people do. Without that context, you can see what happened but rarely why, and so what comes out clearly are the big, objective moments but not the details of people's lives.

All you need to do is ensure your Oracle isn't a kind of cultural Babel fish - people will do the rest for you.