Oi may refer to:
In Japanese geography:

• Oi, Fukui
• Oi, Kanagawa
• Oi, Saitama
• Oi District, Fukui
• Oi River, Shizuoka Prefecture
• Oi River (Kyoto Prefecture), part of the Katsura River

Oi or OI may refer to:
In biology:

• Great-winged petrel, also known by its Maori name oi
• Orthostatic intolerance, a disorder of the autonomic nervous system
• Osteogenesis imperfecta, a genetic bone disorder

Oi! is a subgenre of punk rock that originated in the United Kingdom in the late 1970s. The music and its associated subculture had the goal of bringing together punks, skinheads and other working class youth.