Here is a simple proposal:
A being x is omnipotent provided that in every possible world, x’s free choices are collectively the ultimate explainers of the rest of contingent reality.
In particular, only a necessary being can be omnipotent. Whether omnipotence is compatible with created free will depends on how exactly we spell out “ultimate explainers”. We might think that if y in situation S freely chooses to A, and God creates y in S, and y freely chooses to A, then God’s creation is an ultimate explainer (it may or may not be the case that an ultimate explainer of a proposition is an explainer of the proposition).
This definition is incompatible with Molinisms on which God is not an ultimate explainer of conditionals of free will.
If the above account is right, we have a sound ontological argument along the lines of the standard S5 ontological argument:
- Possibly, there is an omnipotent being.
- Therefore, there is an omnipotent being.