What is open interest in options?

How a trade changes open interest

Every option trade has two sides, and each side is either opening a new position or closing an existing one. If both sides are opening, a new contract comes into existence and open interest rises by one. If both sides are closing, a contract disappears and open interest falls by one. If one side opens and the other closes, the contract simply changes hands and open interest does not change.

That last case is common. A trader who bought calls last week and sells them today to a new buyer leaves open interest exactly where it was, even though the trade shows up in volume. This is why volume and open interest can tell very different stories about the same contract on the same day.

Exercise, assignment and expiration also reduce open interest. When an option is exercised, the contract is extinguished and replaced by a stock position. When it expires worthless, it simply ceases to exist.

When open interest is reported

Open interest is not updated trade by trade. The clearinghouse tallies positions after the close, and the figures are published before the next session opens. So the open interest you see during the trading day is yesterday's ending figure, and it does not reflect anything traded today.

That lag matters in practice. On a day with heavy trading in a contract, you cannot see during the session whether those trades opened or closed positions. You find out the next morning, when the new open interest is published. Intraday positioning tools have to estimate that change from the volume and from where trades printed relative to the bid and ask.

It also matters around expiration. Positions closed on expiration day only drop out of the published figures after the close, and the post-expiry figures appear the next business morning, which is usually the first clean view of the board without the expired contracts.

What open interest tells you

At its simplest, open interest shows where positions exist. Large open interest at a strike means many contracts are outstanding there, which means many hedges are attached to it. That is why dealer positioning models weight every calculation by open interest: it is the best public measure of how much risk sits at each strike.

It also tells you about liquidity. Contracts with very small open interest often have wide bid-ask spreads and can be hard to exit at a fair price. Many traders look for open interest in the thousands before taking a position in a contract, though the right threshold depends on the underlying.

Changes in open interest from one day to the next are often more informative than the level. A strike whose open interest jumps overnight saw new positions opened. A strike whose open interest falls saw positions closed. Paired with the price action, that helps distinguish new money from positions being taken off.

Open interest and the gamma map

Every gamma exposure figure is built by multiplying each contract's gamma by its open interest. That makes open interest the weight on every level a gamma map draws. A strike with large gamma per contract but almost no open interest contributes nothing; a strike with modest gamma per contract and enormous open interest can dominate the board.

It also explains why maps change overnight. When the new open interest is published each morning, every strike's weight is updated at once. A strike where positions were closed on a big day can shrink from a wall to nothing between one session and the next, without a single trade printing in the morning.

What open interest does not tell you

Open interest does not say who is long and who is short. Ten thousand contracts at a strike could be ten thousand customer-bought puts that dealers sold, or ten thousand customer-sold puts that dealers bought, or any mix. Every model that turns open interest into dealer exposure has to assume an answer, and that assumption is the largest source of disagreement between tools.

It does not tell you intent either. A large open interest in puts could be protection on a stock portfolio, a bearish bet, or one leg of a spread whose other leg is elsewhere on the board. Treat it as a count of positions, not a vote on direction.

Every ticker's page at flowmonkey.net/levels shows its call wall, put wall and max pain from the closing book, which are all summaries of the same open interest seen from different angles.