New South Wales
You aren't forced to use one-shot Items you're carrying (i.e., ones you've played to the table).
The relevant rule text:
Munchkin Rules wrote:
A Treasure card that says “Usable once only” is often called a “one-shot”
Treasure. Most of these are used during combat to strengthen the munchkins
or the monsters, and may be played from your hand or from the table. Some
have other effects, however, so read the card carefully! Discard these cards as
soon as the combat is over or their effect is resolved.
Note that it says these may be played from your hand or the table. That's also a 'may'. There's nothing insinuating you must play them at the earliest opportunity.