No OT proponent would suggest that God is *not* omniscient. As I pointed out in a previous post, this is a misrepresentation of OT by its antagonists, partly due to rhetorical posturing, partly due to ignorance of the OT position (whether intentional or otherwise).
Open Theism only "denies" God's knowledge of the future because such denials are necessary due to the entrenchment of "foreknowledge" language within alternative theologies. To the OT, "foreknowledge" is an irrelevant concept, as the future does not exist to be known, and is therefore not an object of knowledge. It is a no-thing, lacking any ontological weight (whether in idea or actuality, or both), and as such cannot be known by anyone or anything, not even God. This, of course, does not represent a "lack" or "inability" within God, as the referent of the "inability" (e.g., the future, e.g., that which does not exist, e.g., "no-thing") is no referent at all. Asking whether God "knows" the future is like asking whether God "knows" what it's like to not exist. The questions are irrelevant and absurd because they misunderstand the most elementary concepts regarding the nature of God's eternal self-existence and knowledge.
But again, these "inabilities" of God are *only* ever brought up by virtue of their contrast to the entrenched language of "tensed divine knowledge" present in other popular theologies; if OT were able to be articulated in a vacuum, apart from the rhetorical dead-weight of these alternative theologies, these "denials" would be completely unnecessary.