What Open Interest Actually Measures
Think of Open Interest like footprints in snow. Volume tells you people walked by today — OI tells you they're still standing there.
Every options contract requires two parties — a buyer and a seller. Open Interest counts the number of contracts where both sides still have an open obligation. When a new buyer and a new seller transact, OI rises by one. When an existing holder closes their position with another existing holder, OI falls by one. When a new trader buys from an existing holder exiting, OI stays flat.
This is why OI tells you something volume cannot: it captures commitment. A strike with 50,000 open contracts has 50,000 reasons for price to gravitate there — each represents a participant who has skin in the game and will react as price approaches.
As expiration approaches, open interest at heavily populated strikes becomes a gravitational force. Market makers who sold those options must hedge their exposure by trading the underlying — and that hedging activity physically moves price toward the strike. This is the mechanics behind what traders call "pinning."
OI as Support and Resistance
Large put OI below the current price functions as a support zone. The institutions holding those puts are often portfolio managers hedging equity exposure. If price falls toward that strike, they may buy back shares to reduce delta risk — creating real buying pressure at that level. The reverse is true for large call OI above price: market makers short those calls will sell stock as price rises, capping the move.
This dynamic is most reliable when OI is dominated by a single expiry. Mixed OI across many expiries creates less concentrated hedging pressure and weaker support or resistance.
Rising vs. Falling OI
When a price move is accompanied by rising OI, it means new money is entering in the direction of the move — a healthy, confirming signal. When price moves on falling OI, it suggests position unwinding rather than fresh conviction. The best trending moves typically see OI rise consistently as price extends, with participants adding to positions rather than exiting.