It stands for market-on-open order; an order that is intended to be executed at the opening price in the first session in a trading day. The order is automatically submitted at the market’s open (or in a very close window of time), and will be executed a the market price available at the time. A market-on-open (MOO) order combines a market order with the open price gap time in force (OPG TIF) to create an order that is automatically posted at the market’s open where filling takes place at the market price.

Practically, this type of order is placed when the market is closed for trading in a given day, but remains resting (queued or parked) in the order book until the open next day. When the market opens the order is automatically submitted to a live auction to be filled at the open price.