A good example of how the author expresses how Okonkwo is by his actions is right at the beginning of the novel:
Okonkwo was well known throughout the nine villages and even beyond. His fame rested on solid personal achievements. As a young man of eighteen he had brought honour to his village by throwing Amalinze the Cat.
An example of how he describes Okonkwo through the narrator's words is the following, from Chapter 4:
Even Okonkwo himself became very fond of the boy - inwardly of course. Okonkwo never showed any emotion openly, unless it be the emotion of anger.