One can read the works of Arminians such as Arminius, Wesley, Miley, or Richard Watson and would be hard pressed to find open theism in their writings.
But one can find Open Theism in the works of Richard Watson. Watson seemed to be a neo-molinist version of an Open Theist. God knows all possible futures, but only sees which are actualized during the present and responds accordingly:
"It may therefore be certainly concluded, that the omniscience of God comprehends his certain prescience of all events however contingent; and if any thing more were necessary to strengthen the argument above given, it might be drawn from the irrational, and, above all, the unscriptural consequences, which would follow from the denial of this doctrine. These are forcibly stated by President Edwards: – “It would follow from this notion, (namely, that the Almighty doth not foreknow what will be the result of future contingencies,) that as God is liable to be continually repenting what he has done; so he must be exposed to be constantly changing his mind and intentions as to his future conduct; altering his measures, relinquishing his old designs, and forming new schemes and projections. For his purposes, even as to the main parts of his scheme, namely, such as belong to the state of his moral kingdom, must be always liable to be broken, through want of foresight; and he must be continually putting his system to rights, as it gets out of order, through the contingence of the actions of moral agents: he must be a Being, who, instead of being absolutely immutable, must necessarily be the subject of infinitely the most numerous acts of repentance, and changes of intention, of any being whatsoever; for this plain reason, that his vastly extensive charge comprehends an infinitely greater number of those things which are to him contingent and uncertain. In such a situation be must have little else to do, but to mend broken links as well as he can, and be rectifying his disjointed frame and disordered movements, in the best manner the case will allow. The supreme Lord of all things must needs be under great and miserable disadvantages, in governing the world which he has made, and has the care of, through his being utterly unable to find out things of chief importance, which hereafter shall befall his system; which, if he did but know, he might make seasonable provision for. In many cases, there may be very great necessity that he should make provision, in the manner of his ordering and disposing things, for some great events which are to happen, of vast and extensive influence, and endless consequence to the universe; which he may see afterward, when it is too late, and may wish in vain that he bad known beforehand, that be might have ordered his affairs accordingly. And it is in the power of man, on these principles, by his devices, purposes, and actions, thus to disappoint God, break his measures, make him continually to change his mind, subject him to vexation, and bring him into confusion."'