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