[order about] or [around] {v. phr.} To dictate arrogantly to someone; […]

[ordinary] See: [OUT OF THE ORDINARY].

[OS] "Oh, shit!"

[OSIB] "Open skull insert brain"

[OT] "Off-topic"

[other] See: [EACH OTHER], [EVERY OTHER], [GRASS IS ALWAYS GREENER ON […]

[other fish to fry] {n. phr.}, {informal} Other things to do; […]

[other side of the tracks] See: [WRONG SIDE OF THE TRACKS].

[OTOH] "On the other hand"

[OTP] "One true pairing", meaning two characters believed to be perfect […]