Options open interest is the number of option contracts that remain outstanding rather than having been closed, exercised and assigned, expired or otherwise terminated. It is normally measured for a specific option series—meaning a particular underlying, call or put type, strike and expiration—and can also be aggregated across strikes, expirations or an underlying. Each outstanding contract represents both a long position and a corresponding short position, but open interest counts that matched contract once, not twice.
Why trading volume and open interest can move differently
Open interest originates from trading, but it is not a measure of how many contracts traded during the session. What matters is whether the buyer and seller are opening new positions or closing existing ones. Consider a single matched trade of one option contract. If the buyer opens a new long position and the seller opens a new short position, a new outstanding contract is created, so open interest rises by one. If instead the buyer is closing an existing short position while the seller is closing an existing long position, an outstanding contract is eliminated, so open interest falls by one. If one side opens a new position while the other side closes an existing position, one outstanding position is replaced by another and total open interest is unchanged.
For example, suppose open interest in a particular call option is 1,000 contracts before a trade. An opening buyer matched with an opening seller increases it to 1,001. A closing buyer matched with a closing seller reduces it to 999. An opening buyer matched with a closing seller—or a closing buyer matched with an opening seller—leaves it at 1,000. In every case, one contract may have traded and therefore added one contract to trading volume, yet the effect on open interest is different.
This distinction also limits directional interpretation. High or rising open interest shows that more contracts are outstanding, but it does not reveal by itself whether market participants are collectively bullish or bearish. Every open option contract necessarily has a long side and a short side. Nor does large open interest at a strike prove that the positions were established for the same reason: outright speculation, hedging, volatility trading and multi-leg strategies can all contribute to the same published total.
Open interest is position data, not a live liquidity quote
For U.S. listed options cleared by OCC, displayed series-level open-interest figures are derived from the previous day's settlement. OCC notes, for example, that figures displayed on a Friday morning represent open interest following Thursday's matching executions. Opening and closing transactions are consolidated in calculating open interest, while exercised options that result in assignment cease to remain open positions. Consequently, the published figure should not be treated as a real-time count of positions during the current trading session.
Open interest can help identify where outstanding exposure is concentrated and can provide useful context for participation, positioning research and expiration analysis, but it should not be used as a stand-alone measure of execution liquidity. A contract with substantial open interest can still have an unattractive bid-ask spread or limited displayed size, while an option with modest open interest can sometimes trade efficiently when active market makers are quoting it. For execution, current bids, offers, quoted size and trading conditions describe the immediate market more directly than the stock of contracts remaining open after prior clearing activity.