One has to define "omniscient" (knowing all things) and "omnipotent" (being all powerful) properly, otherwise one runs into trouble without even considering your question.
Let's deal with "omnipotent" first.
If by "omnipotent" you literally mean "being able to do everything", then consider the following fact:
It is impossible to create an uncreated object.
This is at least one thing that no being, not even God, can do. You may then object that this is irrelevant, but at least you will have to admit that it just makes absolutely no sense to talk about "omnipotent beings" that can do everything because there isn't one.
If you restrict "omnipotence" to "being able to do every possible thing", then you still need to define or axiomatize "possible". One way is via consistency, where we define something to be possible if and only if it can be true without being inconsistent with what are already true. (If you know first-order logic, in a first-order theory possible sentences are simply those whose negation is not provable. But my definition here is not restricted to a particular logic.) If we define "omnipotence" this way, then there is still the following fact:
It is impossible for anything to make it so that nothing ever existed. (English does not have the capability of conveying this accurately, but by "ever" I do not intend any connotation of time but simply actuality.)
This needs some thought, but is undeniable, because even if anything could make everything disappear, he cannot change the fact that something did once exist. Again, I intend this to be not restricted to time-based existence. This has the following implication for any God:
It is impossible for God to make it so that nothing ever existed.
This does not contradict omnipotence of such a God, if we use the restricted definition of omnipotence. But it already tells us one logically irrefutable way in which God is necessarily restricted. However, consider the following:
Is it possible for God to make it so that he/she/it is not omnipotent?
If you say "no", then you would be saying that God is forced to be omnipotent. If you say "yes", then you would be saying that God can choose to become not omnipotent, in which case there can be free will.
Omnipotence and free will
Thus the necessary conclusion from the above reasoning is that the standard argument that omnipotence is incompatible with free will is invalid if the above definition of omnipotence is used. If another definition for omnipotence is used, what would it be? Under most alternative definitions, it's no longer important whether God (if he exists) is omnipotent. So far almost all writings on God's omnipotence do not precisely specify what they mean anyway, so their arguments are not well founded.
I'll use the definition that "free will" means "ability to choose the future". This of course depends on your definition of "future", but it suffices for our argument later. This is also a very weak kind of free will, but it avoids the problem of defining what choices are involved. This also depends on exactly what "choice" means, which cannot really be defined. I'll just say let's use our intuitive meaning for now.
If by "omniscience" you mean "knowing the truth value of everything", then of course any omniscient being will know the future of everything. This applies even in the case that you assume that there is one world for every possibility. In any case, existence of an omniscient being implies non-existence of free will in the sense that we cannot choose the future if it is already something that is known by this being. You cannot say that we can make choices that can change this knowledge because by definition an omniscient being knows the final result of all choices.
Also, a being that is once omniscient cannot choose to become not omniscient, because while omniscient he already knows everything including when he is not omniscient, unless he also can make himself forget what he knew before. But indeed an omnipotent being can become not omniscient and make himself forget what he knew before, so even if there is a God and there is free will we cannot conclude that this God was never omniscient!
Another possible definition of "omniscient" is "being able to determine the truth value of any thing". This is strictly weaker than the above definition of "omniscient", for the same reason that mathematical induction does not imply the existence of an infinite collection. An omniscient being of this kind can still be compatible with free will for choices which he does not attempt to determine until they are made. Again, I intend no time connotation here but English forces my grammar. But most people do not even think of such kind of omniscience, not to say believe that their God (if any) is of this sort.
Some who believe in the existence of a God simply believe that God is omniscient about people in a weaker sense of "knowing what people would do given their choice of virtues". In other words, people have the free will to choose virtues that they desire, but based on those choices their lives are essentially determined. One variant of this is found in the Jewish psalm:
A man's heart devises his way, but YHWH directs his steps.
Different people interpret its meaning differently, but it is consistent with belief in a limited form of free will with a limited form of omniscience.
Reason for creation?
Question: Why would an omniscient God create anything if he already knows what is going to happen? Knowing how it will turn out to be, one does not need to create anything, since creating it is a waste of effort.
Firstly, "waste of effort" may mean two different things to a human and to a God. Even more so, if a God really exists, who created the world and living creatures, and humans can appreciate the (natural) world for its beauty and not just for any material benefit, so also it is reasonable to expect a powerful creator to also be able to enjoy creating and his creation even if he knows how it will turn out.
Secondly, the reasons for doing things usually are much more important than just to find out how things will turn out. We eat food not so that we can find out how they taste, but because we need food. On a higher level, we institute laws and live by them not to find out anything, but because we want to preserve peace and order (social stability). And we want to find out how the natural world works not for curiosity alone, but often also so that we can learn from it to be able to do things better, including feeding ourselves and those around us, and maintaining peace and order.
Thirdly, by the previously mentioned considerations, it is not necessary that belief in a God entails believing in omnipotence or omniscience in the usual sense that people talk about. If for example we have limited omniscience and limited free will in the specific form mentioned above, it is reasonable for God to want to create the creation in order to allow people's choices to be played out, because then choices made would have real consequences that (usually) coincide with the intentions, because the laws governing the world are not chaotic but predictable. So far from predictability being useless, it is in fact extremely meaningful because it gives our choices meaning. Of course, in this view not everything is predictable, such as our choices and those of others.