If you enter a market OPG order you will get in at the open price, however if you aren't apart of the opening price then the order is cancelled and you are not in your trade. If you place a regular market order at the open you will be guaranteed to get in but most likely at an unfavorable price since it is a market order. So with the OPG order you have the opportunity to get the opening price but at the expense of possibly missing he trade altogether if you aren't apart of the open price. With the market order you get in for sure but at the expense of most likely a higher price. Is my understanding accurate? How can you have a higher probability of getting in on an OPG order. If the stock has a high average daily volume, ect.