Oi may refer to:
- Oi (shogi) (Ōi), one of the seven titles of Japanese professional shogi (board game) tournament
- Japanese cruiser Ōi, a WWII era Japanese warship
- Ōi, other name for Japanese artist Sakae, the youngest daughter of Hokusai.
- Oi (telecommunications), the largest landline telephone company in Brazil
- Oi (digraph), a digraph used in various languages
- Great-winged Petrel, also known by its Māori name oi
- Oi (interjection), a British interjection used to get someone's attention, or to express surprise or disapproval
- Oi!, a subgenre of punk rock
- "Oi!", a 2002 hit song for British grime crew More Fire Crew
- Aussie Aussie Aussie, Oi Oi Oi
- Mark "Jacko" Jackson, a former Australian rules football playerer, catchphrase "Oi!"
- Katsushika Ōi, a Japanese painter and print artist and daughter of Katsushika Hokusai
- Oi Oi Oi (album), an album by Boys Noize
- Oi!, a Portuguese language interjection
OI may refer to:
- Owens-Illinois, a Fortune 500 company, a S&P 500 company
- OpenIndiana, a fork (of sorts) of the OpenSolaris operating system
- Optical illusion
- Orthostatic intolerance, a disorder of the autonomic nervous system
- Osteogenesis imperfecta, a genetic bone disorder
- Oriental Institute (disambiguation)
- Orphans International, a global charitable organization the benefits orphans and abandoned children
- Oratorical Interpretation, a forensics category
- gha, a letter, misnamed by the ISO as "LATIN LETTER OI", typographically based on Q
- O-I, a Japanese tank design of the Second World War
- Olympiad in Informatics, or International Olympiad in Informatics, an international programming competition for high school students
- Organizational Intelligence
- Operational Intelligence, a form of real-time dynamic, business analytics that delivers visibility and insight into business operations.
- Oxygen Index, the ratio oxygen:carbon used in Van Krevelen diagrams by petroleum geologists for the assessment of kerogen maturity