When you contact the Oversoul, you initially don't get much of use. The Oversoul, while constructed out of human souls, is considering problems that humans cannot grasp.
Humans are constructed out of single cell, but the concerns of single cells are not really the concerns of the Human. If you imagine a cell being intelligent, it might be concerned about viruses, killer-T cells, internal decay, and the local biochemical environment. The Human is bothered that the light is taking too long to change.
The connection between these two is thin.
So contacting the Oversoul as a whole doesn't really do much for an Oracle, as the Oracle is roughly equivalent to a single cell in the Oversoul. The trick to making Oracular contact useful is a careful context rotation and projection to isolate a single soul.
Ie, you have to project the connection with the Oversoul down to a human level so you can interact with it and get human-level information on it.
The souls that make up the Oversoul still behave differently than independent human souls; they are part of a greater whole, and their purpose (like the cells of a multicellular organism, compared to single celled organism) and motivations remain very difficult to fathom. Still, at the single-soul level, you can get something.
Now, finding a particular soul is ridiculously hard and impractical.
You reach out, and you find a soul. You spend time getting that soul into focus, and viewing it separate from the oversoul. You start exchanging information, and understanding the language of that soul. Only then can you work out, piece by piece, what era, place and name that soul had in life.
If that soul has information you need, you can start communicating it. If not, you get to try again.
Finding one soul out of the trillions dead is impossible. But, finding any soul that died in WW2 is going to happen multiple times per year.
Assuming a trillion souls and 3 hours to connect to a new one and identify it sufficiently, 1000 Oracles working 10 hour days can get through a million souls/year, or 0.0001% of the Oversoul.
Over a century, 10k Oracles can contact 0.1% of all souls in the Oversoul. With 1 in 1000 souls contacted, you can have a really good sketch of history built up.
It gets even better if the Oracles can record how to contact a soul to make it more likely to repeat it. Then Oracle-indexes can be built up over time, and can be used to probe more deeply into historical events after they have been discovered.
This doesn't help with immediate history. Finding one specific dead person? It would literally take 10,000 Oracles a century to have a 1 in a 1000 chance of contacting that person.
This also leads to wonderful plot points. Some events are going to be extremely hard to find, and individual historical figures more-so. The Oracle-index of, say, Alexander the Great is going to be worth a lot. The Oracle-index of a major religious figure is going to be world-shaking.
If there are 10 major religious figures and 0.1% of the dead has been indexed, then the odds any of them are indexed is 1%. For most of them, close disciples are going to be accessible.
This also leads to another thing. If you are interacting with the souls in the Oversoul, do they change? And if they change, can you damage, destroy or harm them?
It would be common that people would claim to have the index of a major religious figure. And insane souls might even believe they are the major religious figure. So I would presume that major religions would claim that their founder's soul "transcended" and is not an individual soul in the Oversoul, and anyone claiming they have contacted them is a heretic, and souls that claim to be the figure are damaged by the adversary and must be purged.