When ties get untied, for short

IN OT

Think of the extra period in sports that happens after regulation ends without a winner—what’s the common two-letter abbreviation for it?

This clue plays on the phrase "ties get untied," which hints at a game ending in a tie that then gets resolved. "For short" signals the answer is abbreviated, and "IN OT" fits—it stands for "in overtime," the extra period in sports when a tied game continues. The wordplay ties (no pun intended) the idea of undoing a knot to undoing a tied score.