Oni (鬼) are ogres or demons in Japanese folklore.

Oni may also refer to:

• ONI (Oxford Nanoimaging), a super-resolution microscope company
• Oni (band), a Canadian progressive metal band
• Oni Press, an independent comic book publisher
• Ooni, the kings of Nigeria's Yoruba people
• Miye Oni (born 1997), American basketball player
• Oni Gozen, a Japanese female warrior from the 16th century
• Oni, India, a small village in Western India
• Oni, Georgia, a town in Georgia
• Oni (letter), the 16th letter of the three Georgian scripts
• Oni: Thunder God's Tale, stop-motion series
• Oni (video game), a 2001 third-person action video game made by Bungie
• Onimusha (series), a series of action video games made by Capcom
• Oni Aliens, foes in the Gantz manga
• Oni, the title of an unfinished Akira Kurosawa script that was later adapted to a video game named Ni-Oh
• Oni, a race of aliens from the manga and anime Urusei Yatsura
• Oni, a primary antagonist in seasons 8-10 of Ninjago: Masters of Spinjitzu
• Oni, an alternate form of Akuma that appears in the Super Street Fighter IV: Arcade Edition video game
• Oni, an antagonist character on the Mortal Kombat: Shaolin Monks video game

ONI may refer to:

• Office of Naval Intelligence, the United States Navy's intelligence agency
• Office of National Intelligence, an Australian government intelligence agency
• OpenNet Initiative, a project for tracking the use of internet filtering and surveillance
• ONI Systems Corporation, an optical networking company acquired by Ciena Corporation in 2002
• Oxygen Not Included, a simulation video game by Klei Entertainment
• OASIS Neural Interface, a VR technology system used in Ready Player Two