It stands for primary only order; a market order that is to be routed as a market-on-open order (MOO) to the primary market for participation in its opening or re-opening process. The order must bet filled at the first published price of the trading day.

It is posted for execution at the day’s opening price and can only be filled when the market opens or very shortly after opening.

A primary only order (PO) posted for participation after the primary market opens is routed and executed as a normal market order. It combines a market order with the only opening (OPG) time in force (OPG TIF) to initiate an order that is automatically submitted at the market’s open and executes at the market price.