The short answer
Open interest for US-listed options updates once per trading day. The Options Clearing Corporation (OCC), the central clearinghouse for listed equity and index options, counts open contracts after each session closes and posts the figure the next morning. The Options Playbook puts it plainly: open interest "is officially posted by The OCC the morning after any given trading session," and for the rest of that day "the figure remains static" (Options Playbook). Market Chameleon describes the same cycle from the data side: open interest is "calculated once a day by the OCC after the close of the trading day" and "made available at the start of the next trading day" (Market Chameleon).
Two timings are worth keeping apart. The OCC publishes the count the next morning. The screen you trade from may pick it up later than that, sometimes a full day later, depending on how its data vendor loads the file. Both are true at once, and the gap between them is where most confusion comes from.
So the figure on a chain at 10:30 on Wednesday describes positions as of Tuesday's close at best. It will not change again until Thursday's count arrives, however much trades on Wednesday. If you want the concept of open interest itself first, start with our open interest explainer.
What time does open interest update?
There is no single official clock time that every trader sees. The OCC does not compute the figure until the session's trades have cleared, and it makes the result available before the next open. From there, each exchange feed, broker and data vendor loads it on its own schedule.
A post on Alpaca's developer forum lays out how long that chain can be. Open interest is "displayed on the OCC web site" the following morning, but it is "not distributed industry-wide until the next business day morning," so "the open interest data for day 1 will always lag and be available on day 3" (Alpaca community forum). In plain terms:
- Day 1: contracts are opened and closed during the session.
- Day 2, morning: the OCC has counted day 1's closing positions and posts them.
- Day 3, morning: in the slower distribution path that post describes, day 1's count reaches the wider vendor network.
Most retail platforms show the new count at or near the next open, but the honest answer to "what time" is "check the date on the figure, not the clock." A screen that says it refreshed a moment ago is telling you when the page loaded, not which session's positions it is showing.
How the overnight count is built
The count is an end-of-day reconciliation, not a running tally of trades. The Options Industry Council, the OCC's education arm, explains that the OCC consolidates trade reports from the exchanges that "detail all opening and closing transactions for the day," and it also tracks options that are exercised and assigned, which are closed and "eliminated from open interest" (Options Industry Council).
Three inputs move each contract's figure:
- Opening transactions. Each order is marked "to open" or "to close." When both sides of a trade are opening, a new contract exists and open interest rises by one.
- Closing transactions. When both sides are closing, a contract is extinguished and open interest falls by one. When one side opens and the other closes, the contract changes hands and the count does not move.
- Exercises and assignments. An exercised contract leaves the count, and the writer assigned against it is released with it.
Those open and close marks are why the count has to wait. A single trade on the tape does not say which of the three cases it was. Only the clearing records, matched across every exchange and clearing member, can net them correctly, and that matching runs after the close.
Why open interest stays still while volume climbs
Volume and open interest answer different questions. Volume counts how many contracts traded today, and it starts accumulating with the first trade of the session. Open interest counts how many contracts remained open at the last close. One is a flow, the other is a stock.
Because the stock is only recounted overnight, a contract can trade 40,000 times on Wednesday (an illustrative figure) and still show Tuesday's open interest all day. Those trades might have opened 40,000 new positions, closed 40,000 old ones, or, far more often, done some of each along with a lot of hand-changing. Nobody outside the clearing process can tell which until Thursday's figure posts.
That is why volume above open interest is a useful flag but not a conclusion. It says today's activity is large relative to standing positions; it does not say positions grew. The next morning's count is where that gets answered, and our guide to change in open interest covers how to read the difference between two of those counts. For the side-by-side comparison, see options volume vs open interest.
A useful habit: whenever a screen shows open interest, assume it is from a prior close unless the screen says otherwise, and look for the as-of date that proves it.
What expiration does to the count
Expiration is the largest single event in the open interest calendar, and it is where the overnight cycle surprises people most.
When a series expires, every remaining contract in it is either exercised or lapses, and either way it leaves the count. The figure posted the morning after an expiration can therefore drop sharply for the whole underlying even though nothing unusual traded. Nobody decided to close those positions; they ran out of time.
Two consequences follow:
- The morning after a monthly expiration often shows a much thinner book than the prior session did, because the third-Friday series is gone. Comparing that count to the one before expiration without adjusting for it compares two different sets of contracts.
- Positions opened and closed on expiration day leave no trace. A same-day contract bought at 9:45 and expired at the close is created and extinguished inside one session, so it never sits in any overnight count. The series itself still carried the open interest it had from earlier sessions; what goes unrecorded is the expiry-day trading. On index products that is a large share of activity: Cboe reported that 0DTE contracts made up 60% of total SPX volume in September 2025.
If that much of an index's daily trading never feeds a later count, open interest describes the standing inventory of positions carried overnight, not the whole of what traded. Keep that in mind every time a level is built from it.
Why different screens show different numbers
Ask three platforms for the open interest on one contract and you can get three answers on the same morning. Usually none of them is wrong. They are standing on different vintages.
The OCC figure is the source, but most traders read it through an exchange feed, a broker or a data vendor, each of which picks up the overnight file on its own schedule. As the forum post above describes, a figure that is current on one screen can be a session older on another. Weekends and market holidays stretch the gap: Friday's close is counted over the weekend, and that figure is what every screen shows until the next session's count arrives.
The fix is to read the date, not the number. A careful data page labels open interest with the trade date it describes rather than the time the page last refreshed. If you cannot find which session's close a figure belongs to, treat its age as unknown. When two downstream sources disagree, the OCC's own open interest reports, listed in the Options Industry Council market data library, are the place to settle it.
Why two GEX dashboards disagree before the open
Gamma exposure (GEX) models multiply each contract's gamma by its open interest. Every GEX figure, free or paid, therefore stands on an overnight count and inherits the vintage problem above.
Early in the morning, before the new count has reached every vendor, one dashboard may already be using last night's figure while another is still on the night before. Both can be computed correctly and still disagree, because they are counting different books. After an expiration the gap can be large, since one model still includes a series the other has already dropped. Our page on reading SPX net GEX today breaks a "today" GEX number into its timestamps, of which the open interest vintage is the slowest-moving.
SquawkFlow works under the same constraint and says so on the page. The SPX GEX dashboard names the settlement date of the open interest its levels are built from, and the overnight open interest change page compares two completed settlement snapshots and refreshes once per session, before the open. Spot price and implied volatility move during the day; the positioning input does not, and no honest tool can claim otherwise.
A quick checklist for reading open interest
- Find the as-of date. Open interest describes a prior close. If the source does not name the session, its age is unknown.
- Do not read volume as new positions. Today's trades show up in open interest only after the next overnight count.
- Mark expirations. A drop the morning after an expiry is mostly contracts running out, not traders closing.
- Remember what is never counted. Contracts opened and expired on the same day never enter an overnight figure, and on index products that is a large share of trading.
- Compare like with like. Before calling one screen wrong, check that both are on the same session's count.
Open interest is one of the most useful numbers on an option chain precisely because it is settled and reconciled. The price of that reliability is that it always trails the tape by at least a session. Read it as a snapshot of positions at a prior close and it will not mislead you.
Educational content, not financial advice. See our risk disclosure.