Opening Imbalance Only Order (OIO)

The U.S. stock market closes at 4 pm EST. It reopens the following trading day at 9:30 am EST. There is some after-hours and pre-market trading, but the opening and closing create bottlenecks when a lot of orders are piled up, waiting to get filled.

The opening imbalance only orders are meant to provide liquidity at the opening bell for the NASDAQ. They help match other open-only orders set by other traders.

The OIO deals are limit orders, meaning they have a specific price assigned. In other words, they aren't market orders that fill no matter what the price of the stock happens to be. The transactions only complete if their price is reached at the open (they tend to be priced either aggressively high or aggressively low) and are there to fill in when imbalances occur.