O6 or O-6 may refer to:
- LNER Class O6, a class of British steam locomotives
- O6 star, a subclass of O-class stars
- OceanAir IATA airline designator
- O-6, a pay grade in the US uniformed services:
- the "Occupied 6", an Irish republican euphemism for Northern Ireland
- USS O-6 (SS-67), an O-class submarine of the United States Navy