British & Australian, humorous

a cradle-snatcher - someone who has a romantic or sexual relationship with a much younger partner.

Pete's new girlfriend's only 15. I'd call that cradle-snatching.

• on end
• one-night stand
• one of a kind
• one of the boys
• one of the lads
• one of those days
• one-shot
• one step ahead
• One step forward, two steps back.
• one swallow doesn't make a summer
• one thing after another
• one thing leads to another
• one-to-one
• one-two punch
• one-upmanship
• on everyone's lips
• one way or another
• one way or the other
• one-way ticket to
• one-way ticket to something
• on feet
• on file
• on good side
• on good terms
• on good terms with