An OCO order consists of a stop and a limit order. When either the stop or limit price is reached and the corresponding order is executed, the other order is automatically canceled.
Keep in mind that an OCO order gets canceled automatically only if the other order is partially or fully filled by market price movement.
If you want to cancel one part of the OCO order manually, you also need to manually cancel the other part.
To place an OCO order on CEX.IO BROKER, go to the trading terminal and click on Buy or Sell in either the Trade or Watchlist widget. In the new window, choose OCO order. Enter your lot size, specify the stop and limit prices, and hit the Send Order button.