What OPEX means
OPEX is trader shorthand for options expiration, most often the monthly expiration on the third Friday, when the largest block of listed open interest in the market comes off the board at once. Options now expire on almost every trading day, but the third Friday still concentrates far more contracts than any daily or weekly expiration, which is why it retains a name of its own. The OPEX calendar 2026 lists those dates with the exchange source behind each one.
The same abbreviation means operating expenditure in company accounts. This page is only about the trading sense.
The monthly and quarterly calendar
Standard monthly equity and index options expire on the third Friday of each month. Four of those twelve are quarterly expirations, and the exchange defines the term plainly: "Triple Witching" refers to "the simultaneous expiration of individual stock options, equity index options and equity index futures that occurs four times a year on third Fridays in March, June, September and December." The exchange adds that on these days "there is often an observed increase in trading volume and market volatility as traders close, roll out or offset their expiring positions."
Note what that claim is and is not. Elevated volume and volatility are directionless. Nothing in the mechanics says which way price goes.
Traders write the monthly one as MOPEX to separate it from the weeklies and dailies. It is the same third-Friday session, not a different product. Triple witching explained covers the four quarterly ones in detail, including why the June 2026 session moved to a Thursday.
What time does OPEX happen
There is no single moment, and the answer differs by product.
Standard third-Friday SPX options stop trading at 5:00 p.m. ET on the preceding business day and settle to Friday morning's opening quotation, so Thursday's close is the last exit. Single-stock options trade through the 4:00 p.m. ET close on Friday itself, and options on broad-based ETFs, including SPY, trade fifteen minutes later to 4:15 p.m. ET. In-the-money contracts that are not closed convert into positions after the close under the clearing house's exercise-by-exception processing.
The settlement detail that catches people out
Standard third-Friday SPX options are A.M.-settled, a change the exchange completed in 1992 after SEC concerns about P.M.-settled index options moving the cash market into the close. Two consequences follow, and both surprise people.
First, these contracts stop trading before their expiration date. Per the SPX contract specifications, "Trading in SPX options will ordinarily cease on the business day (usually a Thursday) preceding the day on which the exercise-settlement value (i.e., the expiration date) is calculated, 5:00 pm ET." You cannot trade out of a standard SPX position on Friday morning. Thursday's close is your last exit.
Second, settlement is not Friday's open or close. It is the Special Opening Quotation (SOQ), which the exchange publishes under the ticker SET. The SOQ is built from the official opening trade price of each of the index's constituents, and those stocks do not all open at once. Because the SOQ is assembled from opening prints rather than a continuous index calculation, it can land outside the day's traded range entirely. The exchange measured this: across quarterly A.M.-settled expirations from March 2009 through March 2024, SOQ values "fell outside the high and low range of the S&P 500 Index... approximately 30% of the time, most often above the daily high." The full detail is in the exchange's paper on settlement of standard, A.M.-settled S&P 500 index options.
What actually changes when the gamma expires
The durable, mechanical effect of OPEX is subtraction. A large block of open interest simply ceases to exist on Friday, and every hedging obligation attached to it disappears with it.
Before expiration, contracts clustered at heavily-traded strikes can anchor price, but only when the hedgers at those strikes are long gamma: a long-gamma book sells as price rises above the strike and buys as it falls below, and that hedging grows more active the closer spot sits to the strike and the nearer expiration comes. Hedgers who are short those options do the reverse, buying strength and selling weakness, which pushes price away from the strike. That long-gamma condition is what the pinning models in gamma pinning require, and it is the mechanism usually offered for the pull described by max pain theory. Pin risk is a separate thing: the assignment uncertainty facing someone short an option that finishes at the strike. The SPX max pain strike for each listed expiration is free to check against spot in the sessions running into an OPEX. After expiration, that anchoring is gone. A level that had held for three weeks can stop functioning the following Monday, not because sentiment shifted but because the open interest creating it was retired.
This is also why post-OPEX weeks can feel different. Whatever dampening or amplifying effect the expiring positioning was exerting is removed, and the remaining book is smaller and differently distributed until new positions accumulate.
One caution matters more than any of this. Dealer positioning is an assumption, never an observation. Open interest tells you a contract exists; it never tells you which side a dealer is on. Every OPEX narrative built on "dealers are long gamma here" rests on a sign convention that may be wrong at any given strike.
Is OPEX bullish or bearish?
Neither, reliably. The honest answer is that OPEX changes a constraint rather than a direction. Beware explanations that assert a seasonal edge without publishing a hit rate, the interesting question is not whether a level exists but how often it has held when price actually tested it. We publish no such rate on our free SPX gamma exposure page, for that exact reason: the one we showed until August 2026 counted the days price never came near a level alongside the days it genuinely tested one. The levels are there with the time they were captured, and the glossary covers the zero gamma level that OPEX so often relocates.
Educational content, not financial advice. See our risk disclosure.