[onion] See: [KNOW ONE'S WAY AROUND] (2).

[only] See: [HAVE EYES ONLY FOR], [IF ONLY].

[OOC] "[Out-of-character]"; usually in [MMORPG] or [MUD] settings when a player […]

[OOO] "Out of the office"; Used similarly to MIA. "Ken is […]

[Op] Operator, a moderator or moderators of a chat room

[OP] Original poster

[open] See: [EYES OPEN, THROW OPEN].

[open and aboveboard] {adj. phr.} Honest. * /Jacob felt that the […]

[open and shut] See: [CUT AND DRIED].

[open arms] See: [RECEIVE WITH OPEN ARMS].