noun
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.
彼は村の長として尊敬されていた。
He was respected as the head of the village.
noun
the greatest (of all the ...)
Dated usage meaning the foremost or greatest among a group. Often seen in classical or historical contexts.
彼は当代の長と言われた剣士だ。
He was a swordsman said to be the greatest of his time.
Suffix meaning 'head' or 'chief' (e.g., 班長, 社長). おさ is a standalone noun, not a suffix, and is dated.
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.