a serious situation where people could die.

The results of the peace negotiations could be a matter of life or death for people in the war zone. (not used with the)

• 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
• on case
• once again
• once and for all
• Once bitten, twice shy.
• once in a blue moon
• once-in-a-lifetime
• once in a lifetime
• once in a while
• once upon a time
• on cloud nine