OCO stands for "one cancels other." It links two orders so that when one fills, the other is cancelled automatically. On Hyperprop you use it to link your take-profit to your stop, so you never leave a leftover order on the chart after you are out of a trade.
Here is the problem it solves. Say you have a position with a stop to limit your loss, and a separate limit order sitting at your profit target. If your stop fills and closes the trade, that profit-target order is still sitting there. Left alone, it could put you back into the market without you meaning to. That leftover is called an orphan order.
How to use it
With an open position and a separate limit order on your exit side, press the OCO button to link that order to your stop. Now, when your stop fills, the take-profit cancels itself automatically.
The result: when one side of your exit fills, the other disappears on its own. Nothing outlives a flat position.