What Is a Market-on-Open Order?
The price a security last traded at yesterday and the price it starts trading at today aren’t always the same thing, and an order tied to the open is built around that gap.
The short answer
A market-on-open order instructs a trade to execute at the price established by the exchange’s opening auction, whatever that price turns out to be, rather than at the previous day’s closing price or any price visible before trading begins. It has to be submitted before a cutoff deadline ahead of the market’s open, and once submitted, the exact execution price is left entirely to the auction process rather than to the trader.
How the opening auction sets a price
Trading doesn’t simply resume each day at the exact price it left off. Many exchanges run a formal opening auction that gathers the orders submitted before the bell and determines a single price meant to reflect the combined buy and sell interest at that moment, accounting for anything that happened after the prior close — overnight news, earnings, or broader market moves. Market-on-open orders participate in that batch process together, rather than trading individually the way orders typically do once continuous trading resumes.
Why the fill price can look different from the last close
Because the opening price reflects everything that has accumulated since the previous session ended, it can differ meaningfully from where the security last traded the day before. A market-on-open order accepts that gap by design — it’s a bet that participating in the official opening price matters more than knowing the exact number in advance. This is the same basic trade-off found in a market-on-close order, just applied to the other end of the trading day: price control is exchanged for participation in a specific, formally determined auction price.
The submission deadline
Like its closing counterpart, a market-on-open order generally needs to be entered before a cutoff time ahead of the actual open, since the auction has to collect and process orders before it can produce a single clearing price. An order submitted after that deadline typically doesn’t participate in the opening auction and would need to be handled as an ordinary order once regular trading begins, subject to the bid-ask spread and price movement of the moment rather than the auction mechanism.
When the opening price is the point
This order type tends to matter to traders and strategies where the official opening price itself is meaningful — for instance, when performance is measured against an index that’s calculated using opening prices, or when a strategy is specifically built around entering or exiting positions at the start of the trading day rather than at an arbitrary point later on. For a routine trade with no particular tie to that specific reference point, a standard market or limit order placed once the market opens may offer more predictability, since the price is visible before the order is submitted rather than determined afterward by an auction.
The bottom line
A market-on-open order is a tool for aligning a trade with a specific, formally determined starting price rather than chasing whatever price happens to be available first once trading resumes. It offers no control over the exact number, but it offers certainty about participating in that particular reference point, similar in spirit to how a trade settlement date is fixed by process rather than by choice — a trade-off worth weighing against the flexibility of an ordinary order placed once the session is already underway.