One cancels the other order (OCO) definition

A one-cancels-the-other order (OCO) is an order whereby, if one order is executed, then the other order is automatically cancelled. It is used when you want to place two orders at the same time: usually with one going long and the other going short. When market movements cause either order to be filled, the unfilled order is automatically cancelled.

You might place this kind of order when you expect a big move in the market but can’t decide whether it will go up or down. The OCO ensures at least one of your trades will open and move in the direction of the next move.