What is the ozo in the story?

It is a high rank of an elder or warrior, Okonkwo was a great warrior and wrestler, took the ozo title in Umuofia.

Highly accomplished men and women are admitted into orders for people of title such as Ndi Ozo or Ndi Nze. Such individuals receive certain insignia to show their stature. Membership in these orders is highly exclusive, and to qualify an individual needs more than mere material accomplishment or gallantry. They need to be highly regarded and well-spoken of in the community, and most importantly, they must be a person of the greatest integrity, truthfulness and sanity. The slightest impeachment of character is enough to disqualify an individual from becoming a person of title and once admitted into the order, a person of title is forbidden to lie, cheat, climb a tree, covet or strip their neighbor of their belongings, or commit an abomination of a crime.