A One-Cancels-the-Other (OCO) order is an order that combines the behavior of a regular limit order with that of a stop market order.
An OCO order has both a limit price and a stop price. On entry, it first behaves exactly like a regular limit order. It can match like a regular limit order, and it contributes to the published market data.
The stop price defines a trigger condition just as for stop orders. Once the trigger condition is fulfilled, the OCO order behaves like a stop market order, i.e. it gets a new priority timestamp and is converted to an incoming market order. The limit price does not apply anymore. When several stop orders and OCO orders are triggered, T7 does not distinguish between stop orders and OCO orders when working out the sequence of processing.
An OCO order that fulfills the trigger condition on entry is rejected by the system. I.e. contrary to stop orders, immediate conversion to regular market orders is in general not supported for OCO orders.
Though the name One-Cancels-the-Other may suggest otherwise, T7 treats an OCO order as one single order, and not as two orders that are linked. This is also reflected by an OCO order having only one Exchange Order ID that does not change throughout its life time, and specifically not when the OCO order is triggered.