Open Interest vs Volume in Options: What Each Column Tells You, and Why GEX Depends on It

Scope: measured on the futures contracts listed (ES, NQ, RTY, YM, GC, SI). Findings apply to the contracts measured and do not automatically transfer to ETFs or other instruments.

Open interest counts option contracts still outstanding after the last settlement; volume counts contracts traded today. One is a stock, the other a flow. A heavily traded strike is not necessarily a heavily held one: if tomorrow's open interest has not risen, the contracts only changed hands. The same distinction splits gamma exposure into two different numbers, which is why a GEX figure means little until its basis is named. If you read either column to decide where the market is positioned, this is the difference between a level that is held and a level that merely traded.

What is open interest, and how does it actually change?#

Open interest is the number of contracts in existence at a given strike and expiry: opened by someone, not yet closed or expired. It is published once per trading day, after the clearing house settles the session, so the figure a reader sees during the day describes yesterday's book, not this morning's.

It moves only when a contract is created or extinguished. Three cases cover everything:

• A new buyer trades with a new seller. A contract that did not exist now exists: open interest rises by one, volume rises by one.
• That buyer later sells to close, and the counterparty is the original short buying to close. The contract is extinguished: open interest falls by one, volume rises by one.
• The buyer sells to close, but the counterparty is a new buyer opening a position. The contract still exists and has only changed hands: open interest is unchanged, volume rises by one.

The third case surprises people. A strike can trade tens of thousands of contracts in a session and end with exactly the open interest it started with, because every trade merely passed existing contracts between parties. Nothing in the volume number tells you which case you are looking at.

How is volume different from open interest?#

Volume is the count of contracts traded during the session. It resets to zero at the start of each trading day, it counts opening and closing trades identically, and it updates continuously rather than once. Where open interest is a level that accumulates across days, volume is a rate measured over one day.

The two fail in opposite directions. Open interest is stale by construction: it cannot show a position built and unwound inside one session, which is most of what happens in same-day-expiry contracts. Volume is blind to history, so a strike carrying a position built over weeks contributes nothing today if nobody trades it.

What does gamma exposure mean?#

Gamma exposure estimates how much of the underlying option hedgers must buy or sell per unit of price movement. An option's sensitivity to the underlying is its delta; the rate at which delta changes as price moves is gamma. Add up gamma across every open contract (the "book"), attach a plus or minus sign based on an assumption about whether the hedgers are net long or net short those options, and the total is GEX.

Every GEX figure is therefore a weighted sum over strikes, and the weight is the contract count at each strike. That is where the two definitions part company: weight by contracts outstanding and you get OI-based GEX, weight by contracts traded today and you get volume-based GEX. Same chain, same gamma per contract, different weights.

Why do the two versions disagree?#

Because they sum different books, and the books have different compositions. OI-based GEX is dominated by accumulated positions across many expiries, including strikes nobody has touched in days. Volume-based GEX is dominated by whatever is trading now, which skews heavily toward short-dated contracts near the money.

Magnitude differences are easy to notice. Sign is the one that matters, because sign carries the qualitative claim: positive gamma is read as hedging that damps movement, negative as hedging that amplifies it. A weighted sum takes its sign from its weights, so a book whose long-gamma strikes (where the hedgers hold options) are old and untraded and whose short-gamma strikes (where they have sold them) are today's active ones can come out positive on one basis and negative on the other, with neither computed wrongly.

Which one should you read, and when?#

Read open interest for where the standing positions sit, and volume for what changed today. Neither is a degraded version of the other. The error is not choosing the wrong one; the error is comparing a number built on one basis against a number built on the other and treating the gap as disagreement about the market.

A provider therefore has to state which basis it uses. Two correctly computed GEX numbers can differ without either being wrong, and the wider set of construction choices behind that gap is covered in why GEX numbers differ.

How would you test which basis tracks price behaviour better?#

Nothing above establishes that either basis predicts anything, and this note does not present a test. Pre-register a single outcome before looking at results: realised movement of the underlying over a fixed window after price first touches the level that basis names, on one instrument across a fixed span of sessions.

The control is the part most published claims omit. Compare each named level against a matched level chosen without reference to the option chain: same day, comparable distance from spot, comparable liquidity, drawn at random from the eligible ladder. A level near current price gets touched constantly because price is near it, so the only question worth asking is whether the named level behaved differently from the matched one, in the direction stated in advance, often enough to survive resampling. Run that procedure on both bases and the comparison becomes an answer.

Limits#

• Both definitions rest on an assumption about which side the hedgers are on. That sign is inferred, never published, and an error in it flips the interpretation of either basis equally.
• Open interest published once a day cannot describe intraday positioning. Any intraday OI-based series is interpolating between two daily snapshots.
• Volume counts opening and closing trades identically, so a volume-weighted book cannot separate position building from unwinding.
• Both are weighted sums over a chosen universe of strikes and expiries. Change the universe and the number changes while the market does not.

Related reading#

• Call wall and put wall — what the strike concentrations these sums run over are.
• Why GEX numbers differ — the other construction choices behind two unequal honest numbers.
• The gamma flip moves all day — what the sign of the sum is supposed to mark.
• Testing the gamma wall — the control design above, applied.
• Order flow and gamma together — the standing book read against the traded tape.