Open interest

How it works

Volume and open interest come out of the same trades and count different things. Volume is activity inside one session: contracts traded, reset to zero the next morning. Open interest is inventory: contracts that exist right now because somebody opened a position and has not yet closed it, let it expire, or exercised it. The Options Industry Council puts the definition narrowly — open interest "only reflects the total number of open (long or short) option contracts for a given option series that have not yet been closed out".

The consequence is the part that surprises people, and it follows from asking what each side of a trade is doing rather than how big the trade was:

• both sides opening a new position: open interest rises;
• both sides closing an existing one: open interest falls;
• one opening and one closing: volume prints, open interest does not move at all.

That third case is the ordinary one in a liquid series. A position handed from one holder to another is turnover with no change in what is outstanding, which is why a day of heavy volume can end with the same open interest it started with, and why a quiet day of position-building can move it more than a busy day of churn.

Why it is a daily number

Open interest is not a market data field in the sense that a quote is. Nothing on an exchange knows it: it is a clearing figure. For US listed options the clearing house consolidates the day's trade reports from every exchange, pairing opening against closing transactions, and only then can it say what remains. The OIC states the timing directly — new open interest can only be reported after clearing and pairing opening and closing positions at the end of the day.

Futures work the same way and are cleared by their own exchanges, and the pattern to expect there is a preliminary figure published soon after the close and a final one the next morning. That is a venue-by-venue schedule rather than one rule, so read the publishing exchange's own bulletin page before assuming a figure is settled; two published numbers for one session, the first revisable, is the normal case rather than an anomaly.

So a chain that shows a bid, an ask and an open interest is showing you two quantities on very different clocks: the quote is now, the open interest is a reconciled count from a previous session. That is not a defect in the feed. It is what the number is, and a reader who assumes the whole row shares one timestamp will draw a conclusion the data does not support.

Why it matters here

Three things to check on a card before relying on the field.

Which session it belongs to. Whether the open interest on a snapshot is last night's or the night before decides whether it can be lined up against a day's prints at all. Several APIs in market data APIs return it inside a live chain without saying, and how to get an options chain covers what free and cheap chains carry.

Whether it is the clearing figure or a derived one. Anything that updates intraday is inferred from trade prints rather than reported, and an inference from the tape has an accuracy nobody publishes. Unusual Whales sells the options tape and positioning measures, which is a different object from a cleared count, and the two should not be described with the same word.

What happens to the series at a corporate event. A split or a distribution can change the contract itself — strike, deliverable, multiplier — and history that carries open interest through an adjustment without noting it is comparing two different instruments. The corporate action entry is the general form of that problem, and point-in-time is the discipline that keeps it visible.

For history, open interest is an ordinary end-of-day field rather than a tick stream, which is part of why options archives are sold as daily files: Cboe DataShop and FirstRate Data both sell it that way, and the rest of the asset class sits in options data. It is also the field most often quoted as evidence of liquidity, which the OIC's own FAQ flags as a common misconception — it says how many contracts exist, not how easily one can be traded, and the NBBO is closer to the question people are usually asking.