Open interest

The number of option contracts currently open at a given strike — a measure of how much a contract is used.

Open interest is the number of option contracts of a given strike and expiration that are currently held open, meaning they have been opened but not yet closed, exercised or expired. Unlike volume, which resets to zero every morning and counts every trade that day, open interest carries over from session to session and only changes when contracts are actually created or wound down.

Traders mainly use it as a read on liquidity. A call with 8,000 contracts of open interest will usually have tighter bid-ask spreads and fill more easily than one showing 12. Say the AAPL 200 call has 25,000 in open interest while the 202.50 shows 40: you can trade the 200 at close to mid, whereas the 202.50 may cost you a wide spread to get in and out of.

A common mistake is treating rising open interest as automatically bullish or bearish. It only tells you positions are being built, not which side is the aggressor. Pair it with volume and price action, and remember high open interest at a strike often marks a level where a lot of hedging activity is concentrated.

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Educational use only. Quotes are delayed ~15 minutes and nothing here is financial advice. Options trading involves substantial risk of loss.