o'

preposition

1. an abbreviated form of of, as in o’clock or will-o’-the-wisp.
2. an abbreviated form of on.

1. a prefix meaning “descendant,” in Irish family names: O’Brien; O’Connor.

preposition

1. informal, or archaic shortened form of of a cup o’ tea

as a prefix in Irish names, from Irish ó, ua (Old Irish au) “descendant.”