From Traderpedia

Definition: A market term where you have two orders, one above and one below the current market price and where the first to be executed automatically cancels the other.

This is a type of order, usually known as a bracket order, which allows two orders, usually a stop order and a limit order, to be placed simultaneously. As soon as one of the orders is filled, the other will be cancelled. One Cancels All (OCA) orders work the same way, except there are more than two orders simultaneously submitted and the triggering of any one will cancel all the others.