O
- Ob-
- Obfuscated C Contest
- obi-wan error
- Objectionable-C
- obscure
- octal forty
- off the trolley
- off-by-one error
- offline
- ogg
- -oid
- old fart
- Old Testament
- on the gripping hand
- one-banana problem
- one-line fix
- one-liner wars
- ooblick
- OP
- op
- open
- open source
- open switch
- operating system
- operator headspace
- optical diff
- optical grep
- optimism
- Oracle, the
- Orange Book
- oriental food
- orphan
- orphaned i-node
- orthogonal
- OS
- OS/2
- OSS
- OT
- OTOH
- out-of-band
- overclock
- overflow bit
- overrun
- overrun screw
- owned