[nutty as a fruitcake] {adj. phr.}, {slang} Very crazy; entirely mad. […]

[NVM] "Never mind"

[NWS] "Not work safe"; see [NSFW].

[Nyo] A usually meaningless word/particle attached to the end of sentences, […]

[O] "Oh"

[oak] See: [GREAT OAKS FROM LITTLE ACORNS GROW].

[OAR] On a roll

[oars] See: [REST ON ONE'S OARS].

[oath] See: [TAKE OATH].

[oats] See: [FEEL ONE'S OATS], [SOW ONE'S WILD OATS].