OCO pairs are most often a stop-loss and a take-profit on the same position. Whichever triggers first closes the trade and the other is removed, so you are not left with an unintended order that could open a new position.
They can also be used for entries: a buy stop above a range and a sell stop below it, so you trade the breakout in whichever direction it comes.
Example: long 100 shares at $50 with OCO stop at $48 and limit sell at $55. Price hits $55, the limit fills, and the $48 stop cancels itself.
Related: bracket-order, stop-loss, take-profit, range