Okonkwo's journey does not exactly follow the classical hero cycle but it is close enough that one can describe him as a classical hero and connect the events of his life to the cyle as it is described or followed in classical myths.
Okonkwo starts with a series of challenges and meets them head on, he works hard to build his fortune, gains titles, becomes a great wrestler, and gains the respect of the tribe. In this he seems to have favor from the gods.
Then he faces his challenges and the problem of his temper which leads to him killing Ikemefuna and angering the gods and having to remove himself to his mother village. This follows the cycle that suggests some type of death or rebirth or transformation. Once he has atoned for this sin or mistake, he is able to return and is welcomed back and celebrated. It would seem that he has returned and completed the hero cycle but this is where perhaps a second cycle begins.
And in this second one, he follows some of the classical mythology that in some ways requires the death of the hero in order to build the hero cult. Okonkwo faces new challenges from his family as well as the incursion of the missionaries and the Christian and Western culture. In this he cannot fight effectively and in the end decides to kill himself as he no longer sees a way forward that would satisfy the gods and his own desire for justice and right.