Omniscience is a hard concept to tackle and when it is tackled(rationally) and brought to the ground it is a total wreck.
Omniscience like omnipotence creates the most absurd paradox in history next to infinities.
If a being(god) was omniscient it would have the ability to know ALL things which is what omni means. Nothing can be excluded, it cannot just perceive and be aware of the choices of all things, it must actually know which choice is going to be made. If omniscience is anything but this it is not omniscience but omni-perception.
To complete this, omniscience must include awareness of the past, future and present. All three modes of existence(or past existence) must become fully aware to the entity. The issue though of course is that it must also be able to know which events will occur and no exceptions can be made. A single exception denies the entity of omniscience.
This odd part about this is that the being MUST know it's own past, present and future choices. Everything this entity does must be known including actions that have not even entered the present.
If this entities entire life and choices are predetermined it cannot maintain free will or the ability to chose. This would also bring into question the moral nature of such a being for when it does make actions and claims to be omniscient.