Literally 'parent role'. The leader for a Yakuza organization. He's the father figure for the clan or house of Kobun, those in the 'child role'. The oyabun is at the top of a pyramid based group or syndicate and exercises total control over those under him. Absolute obedience and loyalty to him are required. All kobun pay him monthly dues and/or tribute payments. Protecting him is of utmost importance. Clan members are expected to sacrifice themselves willingly for his benefit--whether that means going to jail in his stead or dying to protect him.
...if the oyabun says the passing crow is white, then the kobun must agree.1
Leadership skills, enforcement (utilizing violence), and occupational skills (to increase the clan's finances) 3
The retiring oyabun picks a successor, which must meet the agreement of the rest of the clan so there isn't a split in the group. This is the one exception to the oyabun's absolute control. 3
Providing for the needs of those under him via leadership and financial gain through illegal and legal activities
the Boss, 親分 (おやぶん)
Warlord, godfather/boss in Italian mob