- on the block
- on the blower
- on the boil
- on the bone
- on the books
- on the brain
- on the bright side
- on the brink of
- on the brink of extinction
- on the button
- on the card
- on the cards
- on the cheap
- on the chopping block
- on the clock
- on the coattails
- on the committee's docket
- on (the) condition that
- on the condition that
- on the contrary
- on the credit side
- on the critical list
- on the cusp
- on the cutting edge of
- on the cutting-room floor