- on the rampage
- on the back burner
- off one's head
- out of the arc
- of principle
- orenda
- of great moment
- ordained
- oofy
- over one's head
- omnium gtherum
- oceans
- out of one's head
- on your bike
- objurgation
- of one mind
- off-guard
- overwro-ught
- outraged
- oodles of
- old bag
- of note
- off one's own bat
- on the side
- off the deep end
- on easy street
- on one's own initiative
- origins
- of repute
- oneness
- on the sick list
- odd-ball
- of high quality
- old bat
- outré
- one's black dog
- okey-dokey roger
- on a roll
- ocker
- old cow
- odd fish
- on one's tod
- on a short fuse
- on one's uppers
- on the money
- oftentimes
- okey-doke
- opportunistic
- on the button
- on skid row
- of gentle birth
- on one's beam ends
- on the up and up
- old-fogeyish
- off one's rocker
- oft
- off one's face
- oft-times
- orneriness
- oh-so
- odds and sods
- ornery
- optimal
- old hat
- out of one's skull
- overnice
- on the ball
- on the warpath
- one-in-a-million
- one hundred per cent
- oodles
- obliging: couthy
- out of it
- orney
- Oscar
- out of one's gourd
- objurgate
- out of the common
- out to lunch
- overstrung
- off one's trolley
- out of one's tree