- Oxygen rich environment
- Olly olly oxen free
- Over and out
- Out and about
- Out of my mind
- One and done
- One hit wonder
- Orange you glad I didn’t say banana
- Over the rainbow
- On to bigger and better things
- On my way
- Obstinate as a mule
- Ocular degeneration
- Oy vey
- Odd fellow
- Oh my goodness
- Off his rocker
- Other than that
- Out of this world
- Ordinary people
- Olive juice
- Open door
