Source : Webster's Revised Unabridged Dictionary (1913)

O' \O'\ [Ir. o a descendant.]
   A prefix to Irish family names, which signifies grandson or
   descendant of, and is a character of dignity; as, O'Neil,
   O'Carrol.

O' \O'\ ([=o]; unaccented [-o]), prep.
   A shortened form of of or on. ``At the turning o' the tide.''
   --Shak.