Dated term for a leader or head of a group. In modern Japanese, リーダー or 長 (ちょう) as a suffix are more common.
the greatest (of all the ...)
Dated usage meaning the foremost or greatest among a group. Often seen in classical or historical contexts.
He became the head of the village.
He was respected as the head of the village.
He was a swordsman said to be the greatest of his time.
She serves as the head of the clan.
As the head of the town, he brought the residents together.
Standard kanji spelling for this dated noun.
The reading おさ is native Japanese, while the kanji 長 is used for its meaning 'long' or 'leader'. The exact derivation is uncertain, but it has been used historically to denote a person in charge.