What is a Market On Close Imbalance?: Each trading day the major exchanges have to close and set a closing price. The NYSE uses the market on close orders to do that, the Nasdaq uses a crossing mechanism. These orders need to be entered before 15:50 ET after which they cannot be cancelled. A market on close imbalance occurs a larger number of market on close orders on the buy or sell side. If a trader wants to execute a MOC past the 15:50 deadline they must enter an order opposite the imbalance. For instance, if stock A has a buy imbalance then only a sell MOC will be accepted.