OCO orders that are not related to a position can be used to place stop-loss and take-profit orders on a net position, where one is canceled if the other is executed (also known as "bracket orders"). This is useful for example when you have traded in and out in an instrument during the day, and want to protect your net position overnight.
If you do not see the OCO order type, please enable it by going to Advanced Orders (click on the Type field and switch to Use Advanced Orders).