It's difficult to predict how Okonkwo will react because it depends on where you are in the novel and what you know of the events so far. Further, as another reviewer said, his reaction is definitely revealed at the end of the novel.
Early in the novel, we see that Okonkwo is extremely masculine and proud. He is afraid of appearing weak, which is why he is the one to kill Ikemefuna when the young man eventually must be dispatched. He is one of the leaders of the tribe, but his own fellow leaders see him as sometimes too extreme. Because of his leadership position and his personality traits, we can guess that he will try to take action against the missionaries when they come in trying to convert the tribespeople to Christianity. Indeed, he does try to rally his people around the cause, but his violent and aggressive actions do not attract the following he wants.
In the end, the rebellion fails, and Okonkwo hangs himself. On the one hand, it makes sense that Okonkwo would self-destruct and come to a violent end. On the other hand, suicide is sometimes interpreted as a sign of weakness, or that he could not handle the changes in his village. Some might say he just gave up and should have kept fighting. Okonkwo's final decision can be seen as an example of his giving up/giving in, or it can be seen as his ultimate dedication to his tribe and to "the old ways."