On, on, or ON may refer to:
- On (Japanese prosody), the counting of sound units in Japanese poetry
- Ön, a 1966 Swedish film
- On (novel), by Adam Roberts
- Air Nauru IATA airline designator ON
- On in Ancient Egypt, a Hebrew form of the ancient Egyptian name of Heliopolis
- ITV Digital, a failed British digital television service originally named ONdigital
- Leg side, in the game of cricket
- Old Norse, a North Germanic language commonly referenced in etymology
- Operative note, the normally thorough medical documentation of pre-operative, intra-operative, and post-operative care (the pre-op/post-op diagnosis, the surgeons and assisting surgeons and residents and nurses, any complications at any stage, patient name and date of birth, date of surgery, surgical approach, instruments and supplies used, patient's medicines, anesthesia used, primary/any other procedures performed, condition of patient after procedure, consents obtained and explanation given)
- ON Semiconductor, a semiconductor manufacturing company
- O'Nan group, a mathematical group
- Ontario, Canada
- Ontology, a branch of philosophy that deals with the concept of being
- On'yomi, reading of Japanese kanji characters
- Osteonectin, a glycoprotein
- Osteonecrosis
- Order of the Nation, a Jamaican honour
- Overground Network, a railway scheme in South London
- Overmyer Network, an ill-fated US television network
- OS/Net, the OpenSolaris Community
- Optic neuritis, especially relevant in multiple sclerosis
- On, the son of Pelath, participant in an Israelite rebellion against the leadership of Moses
In popular music
- On (band), a musical project by Ken Andrews of Failure
- On (Boom Boom Satellites album), a 2006 album by Boom Boom Satellites
- On (Gary Glitter album), a 2001 album by Gary Glitter
- On (Echobelly album), a 1995 album by Echobelly
- On (Jean album), 2006
- On (EP), by Aphex Twin
- On (Imperial Teen album), a 2002 album by Imperial Teen
- "On", a song by the British band Bloc Party from A Weekend in the City