What is the biblical basis for Open Theism? Open theism, in a nutshell, asserts that the future free choices of agents endowed with free will are unknowable, and therefore that God, though omniscient, is not capable of knowing in advance what we will freely choose in any particular future situation because future free choices are unknowable things, as I just said.
What is the biblical basis for Open Theism?
“Open theism” is an attempt to explain the foreknowledge of God in relationship to the free will of man. The argument of open theism is essentially this: human beings are truly free; if God absolutely knew the future, human beings could not truly be free. Therefore, God does not know absolutely everything about the future.
Open theism holds that the future is not knowable. Therefore, God knows everything that can be known, but He does not know the future.  Open theism bases these beliefs on Scripture passages which describe God “changing His mind” or “being surprised” or “seeming to gain knowledge.”
For example, in the book of Job it seems that God didn’t know what the outcome would be and so he changes his mind.  Through his prophet, God had told Nineveh He would destroy the city in forty days (Jonah 3:4). However, Nineveh repented of their sin (verses 5–9). In response to the Assyrians’ repentance, God relented:

When God saw what they did, how they turned from their evil way, God relented of the disaster that he had said he would do to them, and he did not do it. (Jonah 3:10).

Prior to the flood, it seems like the wickedness of humanity caught God out and he felt regret.  His disappointment is used to suggest that God was not aware of how things would turn out:

And the Lord regretted that he had made man on the earth, and it grieved him to his heart. (Genesis 6:6).

Another instance where it looks like God didn’t know how things would turn out is when Abraham was prepared to sacrifice his only son, Isaac.  God had to test Abraham’s faith:

He said, “Do not lay your hand on the boy or do anything to him, for now I know that you fear God, seeing you have not withheld your son, your only son, from me.” (Genesis 22:12).

After the Israelites offended God by making an image of a golden calf, God’s anger burned against them to the extent he was going to destroy them.  But God changed his mind:

And the Lord relented from the disaster that he had spoken of bringing on his people. (Exodus 32:14).

There may be other Bible verses that the proponents of Open Theism use to support their views but I’ve run out of time for today and must sign off.  You have not asked for a Christian rebuttal so I will simply refer you to a section in the Wikipedia article which criticises open theism and presents a few verses that support the immutability of God: https://en.wikipedia.org/wiki/Open_theism#Criticism