F-09 · Foundations · execution

Market on open: what an MOO order is, and what the open brings with it

An MOO order trades in the opening auction at whatever the opening price turns out to be. How it works, why the open behaves differently from the close, and why so many systems decide at night and act at the open.

By Baris Demirci · 5 September 2026 · Rev00

The idea

A market on open (MOO) order is an instruction to buy or sell at the official opening price, whatever that price turns out to be.

Most stock exchanges start the day with an opening auction. Orders that build up before the market opens are collected and matched at one price, the official opening price. Ordinary trading starts from there.

An MOO order is the morning twin of the market on close order. It carries one condition, which is timing: it trades only in the opening auction. It has no price condition.

Illustration. The four auction orders. Market orders always take part, at whatever price the auction sets; limit orders take part only at their limit or better. This article covers the highlighted one, and the others link to their own articles.

What it promises, and what it does not

It promises to get you in or out at the start of the day. Orders that will trade at any price are matched before orders with limits. On a liquid, widely traded share or fund, in normal conditions, an MOO order fills in full.

It does not promise a price. You find out what you paid only after the auction. At the open that matters more than at the close, for a reason covered next.

Why the open is different from the close

The close ends a day of trading. The open follows a night without it. Everything that happened in between, such as company results, economic data or events abroad, arrives at once in the opening auction.

Illustration, not market data. The open can sit well away from the previous close, because everything that happened overnight arrives at once. An MOO order accepts that gap, whatever its size.

The open can gap. The opening price can sit well away from the previous close, up or down. An MOO order accepts that gap, whatever its size.
You cannot react first. If news lands overnight, the order you placed the evening before still trades at the open, unless you change or cancel it before the cutoff.
The first minutes are rough. After the auction, prices often swing more and spreads are often wider than later in the day. The auction gathers buyers and sellers at one price, which is why an MOO order is usually better than a plain market order sent in the first seconds of trading.
The opening auction is usually smaller. More of the day’s trading happens at the close than at the open. In thinly traded shares the opening auction can be shallow, and one order can move the price.

Rules of the auction you need to know

There is a cutoff. Exchanges stop accepting, changing or cancelling opening-auction orders shortly before the open. In the US that is a minute or two before. Brokers often set an earlier cutoff of their own.
You can place it the night before. Most brokers accept MOO orders after the close or in the early morning, so the order can be waiting before you wake up.
A late open is still the open. If trading in a share is delayed or halted at the start, the order fills at whatever opening price the share eventually gets.
Check that it really goes to the auction. A market order placed before the open is not always routed to the opening auction. Some brokers send it into ordinary trading once the market opens. Ask your broker which happens.
There is a limit version. A limit on open (LOO) order works like a limit on close order, but in the morning: it fills only if the opening price is at or better than your limit. It protects you from a gap, and in exchange it can miss the fill.

Why systems use it

Many mechanical systems read their rules on one day’s close and act at the next day’s open. “Decide on the close, act on the next open” has two attractions:

No peeking. The decision uses only prices that were final when it was made. A backtest built this way cannot use information it would not have had, which is one of the commonest ways backtests mislead.
Time to think. The rule runs in the evening, on final data, with no clock running. The order then waits for the morning.

Illustration of the order of events, not exact times. The rule runs on the final close, the MOO order waits overnight and can be changed until the cutoff, and it fills at the official opening price. Whatever happened overnight is already in that price.

An MOO order is the simplest way to make live trading match a backtest that fills “at the next open”. It is also how a system that buys the close and sells the open gets out each morning.

Where it needs care

Gaps are part of the deal. A system that acts at the open takes on every overnight gap between the decision and the fill. A backtest that fills at the open includes those gaps, so look at how large the worst ones were.
Your data’s “open” may differ from the auction price. Some data records the first trade of the day as the open rather than the official auction price. Usually the two are close. In thinly traded shares they can differ, and the backtest will be slightly wrong.
Not every broker or market offers it. Most large exchanges run an opening auction, but the names, times and rules differ.

The short version

An MOO order trades in the opening auction at whatever the opening price is. It promises the fill, not the price, and the open is where overnight news and gaps arrive. It suits systems that decide calmly on the close and act at the next open. In exchange, you accept the gap and commit before you know what the morning will bring.

Written for people building their own systematic portfolio management process. Nothing here is a recommendation to buy or sell anything. See the disclaimer.

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