Contraction of one is.

Belonging to one.

(slang, MLE) on one's own

(chiefly Canada, US) Involving direct communication or competition between two people. / (sports) Involving one attacker and one defender. / (mathematics) bijective or injective

A contest involving only one player on each side, especially of an activity often involving teams.

Individually in succession; one at a time.

Matching each member of one set with exactly one member of another set. / (chiefly UK) Involving direct communication between two people.

A personal relationship between two people.

Alternative form of one-on-one

plural of one-to-one