What is Open Interest? Definition, Formula, and Example

What is open interest?

Open interest (OI) is the count of options or futures contracts at a specific strike and expiration that exist at the end of the trading day — contracts that have been opened but not yet closed, exercised, or expired. It is a stock, not a flow: if a contract changes hands between two existing holders, OI is unchanged even though volume ticks up. For US equity options, exchanges compute OI overnight from the OCC's morning position file, so the figure displayed during the trading day is the prior session's close.

How open interest changes

Each options trade involves a buyer and a seller, and each side is either opening or closing a position. The combination determines how OI moves:

• Buyer opens + Seller opens: +1 to OI (new contract created)
• Buyer closes + Seller closes: −1 to OI (contract extinguished)
• Buyer opens + Seller closes: OI unchanged (position transfers to a new long)
• Buyer closes + Seller opens: OI unchanged (position transfers to a new short)

Volume = total contracts traded that day. OI = net contracts still alive at the close. When daily volume exceeds OI, new positioning is being built; when volume is a small fraction of OI, the existing book is churning between holders.

Worked example

On 2026-04-17, the SPY June 19 2026 $550 call closed with OI of 45,200. On 2026-04-21 it traded 12,400 contracts of volume, and OI rose to 46,100 — a net +900. Interpretation: 11,500 of the 12,400 trades were position transfers (one side opening while the counterparty closed); only 900 represented net-new contracts. A modest, orderly session.

Contrast: a GME $40 weekly call trades 50,000 contracts on a Monday starting from OI of 2,000 and closes the session at OI of 48,000. That is fresh, directional positioning — retail piling in, dealers writing exposure, and gamma being built into the book. This is the kind of print that feeds gamma squeeze setups where dealer hedging flow snowballs into forced buying of the underlying.

When traders use open interest

• Dealer gamma exposure (GEX) models: aggregate OI by strike, weight by gamma, and infer where market-maker hedging flow will buy or sell the underlying into rallies and selloffs.
• Max-pain analysis: the strike at which aggregate OI value (calls + puts) is minimized at expiration — often a magnet for pin-risk expirations.
• Unusual-activity screens: daily volume greater than prior OI at out-of-the-money strikes signals fresh speculation or informed positioning.
• Liquidity assessment: a contract with high OI tends to have tighter bid-ask spreads and can absorb larger orders without slippage.
• Futures roll timing: commodity and index futures traders watch OI shift between front and back months to time their calendar roll.

Limitations and common misconceptions

Open interest publishes once per day, after the close — it is never truly live. Vendor dashboards showing "intraday OI" display estimates derived from tape analysis, not OCC-confirmed figures. OI tells you how many contracts exist, not who is net long or short direction: a call with 50k OI could represent 50k retail longs facing 50k market-maker shorts, or the reverse — the data does not distinguish. It also does not equal liquidity: a contract can carry high OI and wide spreads if a single holder is dormant. Finally, OI drops sharply at expirations and roll dates by construction — interpreting week-over-week changes without accounting for the calendar produces false signals, especially around the third Friday of each month.

Related terms

• What is a Gamma Squeeze? — the feedback loop that OI concentration can trigger
• What is Options Gamma? — weighting factor for OI in dealer exposure models
• What is Implied Volatility? — priced into every contract counted in OI
• What is Short Interest? — equity-side analogue of OI's positioning signal
• What is a Dark Pool? — where much block equity flow sits invisible to OI analogues