OCO stands for “One Cancels the Other” which allows you to place both stop order and limit order at the same time. Then if any of them executes, the other order will automatically cancel. That’s a great idea for risk migration when you cannot predict the market price of a currency pair whether it will fall down or break above in future.
IFD (if…done) is a kind of pending order consisting of two entry orders at the same time. Then if and only the first entry order is executed, the second order will be activated like a closed order.
IFO is a combination of IFD and OCO. It is similar to IFD, except that the closed order consists of an OCO order.