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Open interest in F&O explained: what it is, how to read it, and why it matters for trading
Open interest in Indian F&O markets explained: how open interest differs from volume, what rising and falling open interest signals about market conviction, how to use it to read option writing patterns and identify strong versus weak price moves.
In one line
Open interest is the total count of active, unsettled futures or options contracts in the market at any point in time. It increases when new contracts are created (a new buyer and a new seller both enter the market) and decreases when existing positions are closed (a buyer and seller who previously had opposing positions both exit). Volume counts transactions in a session; open interest counts positions carried forward.
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Open interest versus volume: the critical distinction
Trading volume counts the number of contracts traded during a specific period. Every trade - whether it opens a new position or closes an existing one - is counted in volume. Open interest counts only positions that are live and carried forward; it is a snapshot of commitment, not activity. Two traders who open new opposing positions increase both volume and open interest. Two traders who close opposing existing positions increase volume but decrease open interest. Two traders where one opens a new position against one who is closing an existing position increase volume but leave open interest unchanged.
This distinction is central to interpreting price moves. A price move accompanied by rising open interest indicates new money entering the market - traders are committing fresh capital to the direction of the move, giving it conviction. A price move with declining open interest suggests short covering or long liquidation - existing positions are unwinding, and the move is not backed by fresh conviction. Experienced derivatives traders always read volume and open interest together: high volume with rising open interest is fundamentally different from high volume with falling open interest even if the price move looks identical on a chart.
How to interpret open interest signals in Indian F&O
In the Indian Nifty and Bank Nifty futures market, the combination of price direction and open interest change gives four primary signals. Rising price with rising OI is the most bullish combination: price is rising and new buyers are adding positions, indicating fresh bullish conviction. Falling price with rising OI is the most bearish: price is falling and new sellers are adding short positions, indicating fresh bearish conviction. Rising price with falling OI suggests short covering - bears are buying back their short positions, causing prices to rise, but no new bulls are committing; the move may lack follow-through. Falling price with falling OI suggests long unwinding - bulls are selling their positions, causing prices to fall, but no new bears are adding; the move may exhaust itself sooner.
For individual stock options, open interest concentration at specific strike prices reveals where market participants expect the stock to stay (or be pinned) - this is the basis of the max pain theory for option expiries. Large open interest at a particular call strike signals significant option writing at that level, which can act as resistance if the underlying approaches it. Large open interest at a put strike signals significant put writing that can act as support. Bank Nifty option chain open interest is closely watched by traders every Thursday before the weekly expiry to identify likely expiry ranges.
Roll-over and open interest at expiry
Monthly futures contracts expire on the last Thursday of the month. In the week before expiry, traders who want to maintain their positions must close their current-month contracts and open next-month contracts - this is called rollover. Rollover percentage measures what fraction of open interest has migrated from the expiring contract to the next month. A high rollover percentage (above the 3-month average) indicates that a large proportion of traders are carrying their positions forward, suggesting conviction in the current trend.
At expiry, all open interest in the expiring contract goes to zero as positions are either settled in cash (for index futures and options) or physically delivered (for single-stock futures). Watching how open interest in the expiring contract falls in the final week - and how quickly next-month open interest builds - helps traders gauge whether the prevailing trend is likely to continue beyond expiry or reverse as unwinding pressure dominates.
FAQ2 reader questions · AEO-eligible
Common questions on what is open interest in derivatives.
Why does open interest sometimes rise even when markets are flat?
Open interest can rise even in a sideways or flat market because new opposing positions are being built. For example, if option writers (sellers) are aggressively writing new puts on a flat market expecting it to stay range-bound, they are adding open interest by creating new contracts. The counterparties buying those puts also hold new open positions. The market can be flat while both bulls and bears are increasing their derivative exposure in anticipation of a future directional move. Rising open interest in a flat market is often a precursor to a breakout: both sides are loaded up, and whichever way the market breaks, the trapped side liquidates rapidly and amplifies the initial move.
What is cost of carry and how does it relate to open interest?
Cost of carry is the premium embedded in futures prices relative to spot prices, representing the theoretical cost of financing a position (interest cost minus dividend income) over the life of the contract. Futures normally trade at a premium to spot - called contango. When futures trade at a discount to spot (backwardation), it signals that participants are willing to accept a lower price to exit spot exposure, often a bearish sign. Open interest in the futures contract with a premium (normal contango) backing a bullish move is considered a higher-quality signal than the same price move with futures trading at a discount or shrinking premium.
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