It stands for open price gap; the time of a trading session that marks its opening or reopening. It is an order qualifier (OPG time in force) that indicates such orders that are directed to the opening auction. If not filled during the opening session, such orders, or part thereof, will expire. And if no uncrossing takes place during the opening auction, these orders will also expire.

An order will be rejected (not filled) if a security does not have a scheduled opening auction.

An example is 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.