In the below example, we've set the over/under at 2.5 rounds for a fight.
This means, you're betting on whether the fight goes more or less than two and a half rounds. That is, two completed rounds, and before (under) or after (over) the half of the third round. If the round is 5 minutes, the halfway is 2 minutes and 30 seconds.
For your convenience, you can view our MMA/Boxing Rules here.