Key takeaway
Open interest (OI) is the total number of futures or options contracts that are still open — not yet closed, exercised, or expired — at a given point in time. Unlike trading volume, it doesn't reset every day, which is exactly what makes it useful for judging whether a price move has real conviction behind it or not.
What Is Open Interest (OI) in the Stock Market
Open interest is the total number of outstanding futures or options contracts for a specific strike and expiry that haven't yet been closed out, exercised, or settled. If 40,000 contracts of the Nifty 24,000 call option are still "live" — held by someone on the buy side and someone on the sell side — the open interest for that contract is 40,000.
OI exists because every futures or options contract needs two sides: a buyer and a writer (seller). NSE publishes OI for every stock and index F&O contract at the end of each trading day, and it shows up right next to price and volume on the option chain of any broker — Zerodha's Kite, Groww, Upstox, or the NSE website itself.
Key takeaway: Open interest tells you how many positions are still open in a contract — it's a measure of participation and commitment, not a prediction of which way the price will go.
How Open Interest Works
OI moves for a simple reason: a contract only exists between a buyer and a seller, and OI only changes when a contract is created or destroyed — not merely when it changes hands.
- A new contract opens. A trader buys, and the counterparty is writing that contract for the first time (not closing an existing position). This is a fresh contract, so OI goes up by that many lots.
- An existing contract closes. A trader who already holds a position sells to exit, and finds a counterparty who is also closing out an existing opposite position. No new contract is created, so OI goes down.
- A contract simply changes hands. An existing position is bought by someone else stepping in as a new counterparty while an existing holder exits — one side opens as the other closes. OI stays the same, because the total number of open contracts hasn't changed.
- 1
Morning: Nifty spot is at 24,000. The weekly 24,000 CE (call option, lot size 65) shows an open interest of 40,000 contracts on the option chain.
- 2
A trader buys 20 lots of the 24,000 CE from a trader writing it for the first time — a brand-new contract is created, so OI rises by 20 lots to 40,020.
- 3
Later, a trader holding 15 lots books profit and sells to close, matched with a buyer who is also closing an existing short position — no new contract is created, so OI falls by 15 lots to 40,005.
- 4
At the close, the option chain shows OI of 40,005 for the 24,000 CE — this running "contracts still open" count is what open interest reports, not the number of trades that happened during the day.
Open Interest vs Volume
Open interest and volume are the two numbers traders confuse most, and they measure genuinely different things. Volume counts how many contracts traded during a single day — it resets to zero every session. Open interest counts how many contracts are still outstanding — it carries forward from one day to the next until those contracts are closed or expire.
| Aspect | Open Interest | Volume |
|---|---|---|
| What it measures | Contracts still open at a point in time | Contracts traded during a given day |
| Resets daily? | No — carries forward until positions are closed or expire | Yes — starts again from zero each session |
| A high number means | Many positions are currently held open | The contract saw a lot of trading activity that day |
| Typical use | Judging whether a price move has fresh conviction behind it | Judging how actively/liquidly a contract is trading right now |
A contract can have huge volume with barely any change in OI — that just means a lot of existing positions changed hands. It can also have modest volume but rising OI, which usually means fewer, larger new positions are being built.
Open Interest Example
The real value of OI shows up when you read it alongside price movement, not on its own. Traders combine the direction of price with the direction of OI to get a rough read on what's happening underneath a move — four combinations, each with a standard name.
Price Up + OI Up
Long buildup — new buyers are entering and are willing to hold, generally read as a bullish sign for that contract.
Price Up + OI Down
Short covering — traders who had bet against the price are exiting, which pushes price up without necessarily reflecting fresh bullish conviction.
Price Down + OI Up
Short buildup — new sellers are entering expecting further downside, generally read as a bearish sign for that contract.
Price Down + OI Down
Long unwinding — existing buyers are exiting as price falls, rather than new bearish positions being built.
Say a Mumbai-based equity investor who's comfortable with stocks but new to derivatives is watching a stock's monthly futures contract. The price rises 3% over two sessions, and OI on that contract rises alongside it by a similar proportion — that combination (long buildup) is a stronger signal of genuine buying interest than the same 3% rise with OI flat or falling, which would suggest the move is thinner than it looks.
Why Open Interest Matters to Traders
OI is mainly used as a context check on a price move, not as a standalone signal. A few concrete uses:
- Confirming whether a trend has backing. A breakout on rising OI suggests new money is entering in that direction. The same breakout on falling OI is more likely short covering or long unwinding — worth noticing before reading too much into the move.
- Gauging liquidity before you trade. Strikes with very low OI tend to have wide bid-ask spreads, which makes entering and exiting more expensive. Traders generally prefer strikes with meaningfully higher OI for that reason.
- Spotting where positioning is concentrated. Strikes with unusually high OI show where option writers have concentrated their bets, which is one input (among several) that traders use when thinking about likely support and resistance zones — not a guarantee of where price will land.
This is also where beginners commonly go wrong: treating a single day's OI change as a signal on its own, without checking what price did alongside it. OI only becomes informative in combination with price — read alone, a number going up or down doesn't tell you which side is winning.
Common Misunderstandings
High open interest on a strike means the price is guaranteed to move toward it.
High OI shows where positions are concentrated, not where price is destined to go — it's a positioning data point, not a forecast.
Open interest and volume are basically the same number.
Volume resets to zero every session and measures activity; OI carries forward and measures how many contracts are still open.
Rising open interest on a contract you're holding means your trade is right.
Rising OI just means more positions are opening on both sides overall — some may be on your side, some on the opposite side.
A contract with more open interest is automatically a better trade.
Higher OI usually means better liquidity and tighter spreads, but it says nothing about which direction the price will move.
This last point matters especially for anyone acting on a stock tip forwarded in a trading group without checking the underlying option chain themselves — "OI is rising" is repeated often in such messages as if it were a buy signal by itself, when it only means something once you also know what price is doing.