OCO stands for One Cancels Other or Order Cancels Other. This is a way to place 2 orders where if one executes the other gets canceled. This can be used if a trader expects price to break out, but isn't certain which direction the breakout will occur in. In these circumstances, the trader could place a pending buy order (buy stabove the current price, a pending sell order (sell stop) below it, and connect the orders as OCO. As soon as one order triggers, the other gets canceled.