Suppose God is omnipotent. Then, it seems, he can bring it about that
(*) a new crater now appears on the far side of the moon if and only if Jones tomorrow freely mows the lawn.
But if God did that, then his knowledge of the present state of the moon, plus his knowledge of his omnipotence, would yield him knowledge of whether Jones tomorrow mows the lawn. Hence, if one restricts omniscience with respect to future free actions, one must similarly restrict omnipotence.
This may not be such a big deal. After all, although (*) is logically possible, the open theist may claim that it is logically impossible that God bring about (*). Still, it does show that there is a connection between omniscience and omnipotence.
One might think that an open theist who holds that propositions about future free actions cannot have truth value, or who holds that reports of future free actions are all automatically false, can escape the worry about the above restriction on omnipotence. After all, if such propositions are all false, then God can bring (*) about simply by doing nothing, since the right hand side of (*) is automatically false. And it seems too much to ask out of omnipotence to require that God bring about a proposition that cannot have a truth value. But one can still modify the task to get around this response. Let the task be to bring it about that:
(**) At t it be true that a day before t Jones mowed the lawn iff two days before t a new crater appeared on the far side of the moon,
where t is two days from now. In other words, the restriction on omniscience still implies a new restriction on what histories God can bring about. Again, it may not be such a big deal to the open theist.