Open Theism like Closed Theism are extreme views, both unbiblical. Closed Theism says whatsoever comes to pass God ordained (predestined) it. In order to know the future, so the Closed theology asserts, God has predestined everything imaginable. God knows the future exhaustively, and the future is predestined exhaustively. Nothing anyone can do will alter the predestined future, you were saved or damned from all eternity for all eternity. Hyper-Calvinists endorse this view whereas main-stream Calvinists say they do not, but in actuality they do. But they hide it in arcane verbiage.
At the other end of the spectrum, Open theism posits the idea that God cannot know the future and is sometimes "surprised" by events. Biblical truth falls somewhere in between these two extreme views which take scripture too far.
So the issue before any bible student is not that God does not know the future, but whether He knows the future exhaustively. The traditional view is that when the Bible says God is "all knowing" the "all" refers to everything imaginable. However, if the "all" simply refers to whatever the author had in view, then those verses do not support exhaustive knowledge of the future. For example Peter says in John 21 that Jesus is "all knowing" but since Jesus did not know the time of His return, the all appears to refer to either "all about Peter" or more broadly, "all about those Jesus encounters." To assert more takes the verse out of its context.
The more modern view is God knows everything He has chosen to know, but this allows that God can choose not to know some things. So He can forgive our sins and remember them no more forever. Those that cling to the exhaustive view say these verses do not mean what they say. Any theology built on nullification is suspect.