American

to be responsible for dealing with arrangements at work or at home while the person who is usually responsible is not there.

So who's going to be minding the store while your manager's away?

• on a knife-edge
• on all fours
• on an empty stomach
• on an even keel
• on a par with
• on a par with someone
• on a par with something
• on a roll
• on a shoestring
• on a silver platter
• on a slippery slope
• on a tangent
• on automatic pilot
• on a wing and a prayer
• on back
• on balance
• on behalf of
• on behalf of someone
• on bended knee
• on bended knees
• on best behavior
• on blocks
• on board
• on borrowed time
• on call