The OP is a total misnomer and misrepresentation, which has been argued and held by several here and rightfully so.
Setting the record straight, Open Theism denies the Omniscience of God, and comes out and states it readily, and is in fact a heresy against the nature and attributes of God.
Omniscience is said to be embraced within Arminian theology, but when the teachings come out, it is denied, using arguments of rationalization, dodging 'the problem of evil', placing God in the 'eternal now' in attempt to justify Him (as if He needs justified!) along with other rationalizing theories that undermine and deny Omniscience.
Omniscience is not only denied in teachings of Armininanism, it is also denied within non-Calvinist theologies as well, as has been seen on here.
Here are a couple of views against Omniscience:
"omniscience need not mean exhaustive foreknowledge of all future events. if that were its meaning, the future would be fixed and determined, as is the past." For them, the idea of foreknowledge "requires only that we define the scope of foreknowledge with care. In some respects the future is knowable, in others it is not. God knows a great deal about what will happen. He knows everything that will ever happen as the direct result of factors that already exist. He knows infallibly the content of his own future actions, to the extent that they are not related to human choices. All that God does not know is the content of future free decisions, and this is because decisions are not there to know until they occur." Frederick Sontag, JETS 1991
The late Clark Pinnock, a noted Open Theist agrees with Sontag here:
"We do not limit God by saying that he can be surprised by what his creatures do. It would be a serious limitation if God could not experience surprise and delight. The world would be a boring place without anything unexpected ever happening" Clark Pinnock, The Openness of God, 123
It is easy to see that God here is reduced to having the passions and desires of man, and that His Omniscience is denied based upon ones logic that He too must desire the element of surprise, and via other finite attributes of man.
This reducing God to an anthropocentric level is one of several reasons many embrace the Doctrines of Grace which accept and reveal God in all His Soveriegnty and Glory well above mankind being wholly other. Reformed theology simply embraces the Scriptures presentation of the Godhead knowing all things perfectly, both past, present, and future.
One may not understand all that is entailed in God's Omniscience, but one fact remains; He is Omniscient being totally aware of all events in history both post and pre creation, and of both the 'good' and 'evil', even using secondary means to accomplish His will and purposes, which He also foreknew, even when said means don't sit well within the logic and reason of finite man. Reducing God due to finite reason is becoming a popularized concept that denies the true God of the Scriptures.
Lorraine Boettner on the subtle denial of Omniscience via Arminianism:
Many Arminians have felt the force of this argument, and while they have not followed the Unitarians in denying God's foreknowledge, they have made it plain that they would very willingly deny it if they could, or dared. Some have spoken disparagingly of the doctrine of foreknowledge and have intimated that, in their opinion, it was not of much importance whether one believed it or not. Some have gone so far as to tell us plainly that men had better reject foreknowledge than admit Predestination. Others have suggested that God may voluntarily neglect to know some of the acts of men in order to leave them free; but this of course destroys the omniscience of God. Still others have suggested that God's omniscience may imply only that He can know all things, if He chooses,---just as His omnipotence implies that He can do all things, if He chooses. But the comparison will not hold, for these certain acts are not merely possibilities but realities, although yet future; and to ascribe ignorance to God concerning these is to deny Him the attribute of omniscience. This explanation would give us the absurdity of an omniscience that is not omniscient.
What we see is that both Arminianism, much of non-Calvinist theology, and Open Theism all share the same sentiments on this doctrine, and they are the ones who are the strange bedfellows in this case, rather, they are familiar bedfellows. As a matter of fact, some notable Arminians recognize Open Theism as resulting from 'freewill theists' (Arminianism) itself, Roger Olson being one, which is sometimes referred to as "Deformed Arminianism" "Hyper Arminianism" among other names, but nonetheless Open Theism is commonly noted as stemming from Arminianism.
Denial of Omniscience then is the outgrowth from freewillism, which is a tenet of Arminian theology. That this stems from the error of freewillism is one proof that Open Theism could never have come forth from Calvinism or Reformed theology, and, that the two are not strange bedfellows, but rather has it's roots from the errant freewill notion borne from Arminian theology. One could also conclude fairly that the trio of Arminian theology, Open Theism, and Socinianism are all three
familiar bedfellows, all three sharing the denial of Omniscience both openly and subtilely.
In conclusion one would be hard pressed to find many Arminians who would come out and implicitly state that God is Omniscient, and knows all things both present, future and past without side notes and issues which they add actually denying this truth altogether.