O line or O-line may refer to:
Tseung Kwan O line, an MTR line in Hong Kong Norfolk Southern O-Line, a freight rail line in North Carolina, United States O-line (IRCd), the operator line of an IRC daemon configuration file O-line, or offensive line, a formation in North American football
See also O-Train, a light rail rapid transit system in Ottawa, Ontario, Canada Orange Line (disambiguation) Online (disambiguation)