OCO orders that are not related to a position can be used to place a stop-loss and take-profit order on a net position, where one is cancelled if the other is executed (also known as "bracket orders"). This 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 OCO order type, please enable it by going to Advanced Orders (click on Type field and switch to Use Advanced Orders).
