It seems the issue here is, in a word, causality. The whole natural world is determined by causalities unless acted upon by something supernatural. Every time you roll a pair of dice the result is determined by the position of the dice as they leave your hand, the speed and torque placed on the dice when they were thrown, wind resistance, gravity, friction between the dice and whatever surface they are being thrown onto, etc, etc. All of which are fixed quantities once the dice actually leave your hand. And so the result is determined the moment that you are no longer in contact with the dice (assuming no one else touches them before they come to rest).
Likewise, chemical processes of all sorts are determined by the chemicals present and the conditions (temperature, pressure, etc) under which the process is running. With one set of chemicals and one set of conditions there is only one possible outcome. If the chemicals and conditions are duplicated, so will the results be. This is because of causality. All natural processes are absolutely determined by causality unless acted upon by something supernatural. There is no such thing as a completely random (i.e. totally without cause) natural phenomenon.
Now, this would seem to be a problem for Open Theism but the reason it is not is not because people are random, that's not it at all! People are not random but they are also not totally natural, they are spiritual and possess a soul. That which is spiritual is not natural but rather supernatural. We are made in the image of God and thus to think that we are completely natural creatures is to go against the clear teaching of Scripture.
The point being is that resorting to randomness to get around the causality problem is not necessary. Causality is strictly natural and does not apply (strictly) to the supernatural and as humans being are only one third natural creatures, causality causes no problem for the Open Theist in any respect. Our will can be free because we are spiritual creatures with a supernatural soul.
Resting in Him,
Clete