What is Open Interest?
Open interest (OI) is the total number of outstanding options contracts at a specific strike price and expiration that have been opened but not yet closed, exercised, or expired. Think of it as the number of contracts still outstanding. Every options contract has a buyer and a seller, so one contract of open interest represents one matched pair.
Open interest is updated once daily, after the market close, when the options clearing house tallies all activity. This differs from volume, which accumulates trade by trade during the session. Understanding the distinction between these two metrics is fundamental to options analysis.
Open Interest vs. Volume
Volume counts how many contracts changed hands today. It resets to zero each morning and accumulates throughout the day. High volume means heavy trading activity but does not tell you whether positions are being opened or closed.
Open interest counts how many contracts exist. It carries over from day to day. If today's volume is high and tomorrow's open interest increases, new positions were opened. If volume is high but open interest decreases, existing positions were closed.
Here is a concrete example. Suppose AAPL $200 calls expiring in 30 days have an open interest of 10,000 contracts at the start of the day, and 3,000 contracts trade during the day (volume = 3,000). If the next open interest figure is 12,000, the net change is +2,000: at least 2,000 of the day's trades opened a position on both sides. The other 1,000 contracts of volume could have been transfers (one side opening, the other closing) or opening trades offset by closing trades, and open interest cannot tell those apart. If the next figure is 8,000, the net change is -2,000: at least 2,000 trades closed a position on both sides (or contracts were exercised), and again the rest of the volume cannot be decomposed from the two numbers.
Why Open Interest Matters
Open interest serves several critical functions for traders:
Liquidity indicator: High open interest at a strike means tighter bid-ask spreads and easier execution. If you need to trade 100 contracts, doing so in a strike with 50,000 open interest is much easier (and cheaper) than in a strike with 500 OI.
Support and resistance: Large open interest concentrations can matter for price near expiration through dealer hedging, which pulls price toward a strike only when the hedgers there are long gamma and pushes it away when they are short. This is the territory of max pain theory: max pain is the strike at which the total intrinsic value paid to option holders at expiration would be smallest. Our free SPX max pain calculator plots that open-interest distribution one expiration at a time, so you can see whether the open interest sits at a single dominant strike or is spread thin across many.
Gamma exposure: Open interest at each strike contributes to the total gamma exposure that market makers must hedge. The GEX profile, call wall, and put wall are all derived from the open interest distribution across strikes. Changes in OI directly change these key levels.
Positioning map: The ratio of call open interest to put open interest at various strikes shows where contracts are concentrated. It does not show direction: every open call has a writer as well as a holder, and heavy call OI above the price can be covered calls written against stock as easily as speculative longs.
How to Analyze Open Interest
The traditional reading pairs the change in open interest with the price move. Each pairing below is an interpretation, because open interest has no side: every contract opened has a holder and a writer.
OI buildup with rising price: New call positions are being opened during a rally. They may be new longs, or calls written against stock; the open interest figure does not say which, and it does not confirm that the rally continues.
OI buildup with falling price: New put positions are being opened during a decline, which may be new hedges, speculation, or puts sold by traders willing to buy lower.
OI decline with rising price: Positions are being closed or exercised during a rally. Short covering is one possible reason; it is not the only one.
OI decline with falling price: Positions are being closed or exercised during a decline. Liquidation of longs is one possible reason.
Trade-side data and the option's own price move are needed before calling any of these bullish or bearish.
Open Interest and Expiration
Open interest becomes especially significant as expiration approaches. Large OI at specific strikes is the precondition for "pinning", where market makers' hedging pulls the stock toward a strike, but that only happens when the hedgers there are long gamma. Monthly OPEX (the third Friday) and quarterly OPEX retire the most contracts at once.
If a stock has large call OI at $100 and large put OI at $90, those strikes mark where the expiring open interest sits. Whether hedging leans against moves toward them or adds to those moves depends on the side dealers hold at each strike, which open interest does not show.
Tracking on SquawkFlow
SquawkFlow builds its gamma levels from daily-settled open interest, showing how the OI distribution produces the call wall, put wall, and GEX levels on its free SPX GEX page. The overnight open interest change page and the dated open interest changes archive show which contracts gained or lost open interest between two settlements, with no direction attached.