Open Interest: What It Actually Counts
Open interest is the number of option contracts at a strike that remain open rather than having been closed out or expired. It measures how much of that strike is still outstanding, which makes it genuinely useful for judging liquidity and entirely useless for judging sentiment.
How it works
Open interest is the number of contracts at a given strike and expiry that have not yet been closed or expired. It rises when a new position is created and falls when one is closed out.
Which makes it a stock rather than a flow. It describes what is outstanding right now, not what happened today.
Volume is the flow. It counts contracts traded during the session, including positions opened and closed the same day. High volume with unchanged open interest means people traded with each other and went home flat.
The two together are more informative than either alone. Volume far above open interest suggests new activity arriving; volume well below it suggests an established position sitting quietly.
What it is good for
Liquidity is the reliable use. A strike with substantial open interest generally has more participants quoting it, a narrower spread, and a better chance of being exited at a sensible price.
That matters more than it sounds. An option position is only worth what it can be closed at, and a thin chain converts a correct view into a bad fill.
It clusters at round numbers. People choose 100 rather than 97.50, which produces concentrations that reflect human preference rather than any view about where price will be.
Very large concentrations can influence the underlying near expiry. Firms hedging those positions trade shares as gamma rises, which is the real mechanism behind stories about price being drawn toward a strike. It is a genuine effect, it is usually small, and it is not a trading signal.
In practice: reading it honestly
Every open contract has a buyer and a seller. The number counts pairs, so a large figure at a call strike does not mean the market is bullish — it means somebody bought and somebody sold, in equal quantity, by definition.
It also does not distinguish a speculative position from a hedge. A large block of calls may be somebody’s directional bet or somebody’s protection against a short position elsewhere, and open interest cannot separate the two.
It is also not updated in real time. Open interest is generally computed after the close, so today’s figure describes yesterday and a comparison against today’s volume is comparing two different moments.
What open interest is not
It is not sentiment. The most common misuse, and it fails on the arithmetic: the number counts matched pairs, and pairs have no direction.
It is not volume. One counts what remains, the other counts what happened. They answer different questions and are frequently quoted interchangeably.
It is not a prediction of where price will settle. The clustering at round strikes is a fact about how people choose numbers.
And it is not comparable across expiries. A monthly contract accumulates open interest over months and a weekly one over days, so the raw figures describe different lengths of time.
It is also not comparable across instruments. A figure that is enormous on a mid-cap share is unremarkable on a major index, so the only meaningful comparison is against other strikes on the same chain.
When it fails
The characteristic failure is inventing a direction. A large figure at an out-of-the-money call strike gets reported as bullish positioning, and it is equally consistent with a large seller.
The second failure is trading the pin. The tendency for price to settle near a crowded strike is weak, unreliable, and competed away by participants faster and better equipped than a retail account.
A third is ignoring it entirely. A strike with almost no open interest can be several percent wide to trade, which is a larger cost than most of the edges people are chasing.
A fourth is comparing calls to puts as a ratio and stopping there. The resulting figure is popular, easy to compute and carries the same problem as the raw number: both sides of every contract exist.
And a fifth is assuming it will still be there. Open interest can fall sharply as positions are closed, so a strike that looked liquid when you entered may be considerably thinner when you want out.
A sixth is reading a single day’s change as new information. Open interest rises when positions are opened and falls when they are closed, and the daily change nets those together — so a flat figure can conceal a large amount of both happening at once.
The reason this number attracts so much interpretation is that it looks like a window. It is published, it is specific, it varies, and it sits next to prices that move. All of that makes it feel like it must be telling you something about intent. It is telling you about size, and size is a fact about how easy the position will be to leave rather than a fact about where price is going.
The original data
5 of the 24,971 videos measured for this site cover open interest, at a median of 1,162 views — a small supply and a low median, on a figure that appears on every option chain and is misread more often than almost anything else there.
The one honest use is the liquidity one, and it is worth having. Before opening any option position, check that the strike has enough outstanding interest that the spread is tolerable and an exit exists. That is a question with a real answer, and it is the only question this number is capable of settling.
Related
Options expiry is where open interest goes. Volume is the flow measure it is constantly confused with. And gamma is the mechanism behind the one real effect large concentrations have.
Open interest is on my screen for exactly one reason: to check whether I can get out of a strike at a sensible price. Every other use I have tried reading into it turned out to be a story I was telling myself.
— Michael Whitman
This page is educational, not financial advice. Test every idea on your own charts before risking money.