Strike Price

The strike price, or exercise price, is the fixed price at which an option contract's holder has the right to buy (a call) or sell (a put) the underlying. Every option chain lists a ladder of strikes around the current price, and where open interest concentrates on that ladder - producing the call and put walls - is one of the central inputs to options market-structure analysis.

A strike price is set when a contract is listed and does not change over its life; what changes is the relationship between the strike and the underlying's current price, described as moneyness. A call is in the money when the underlying trades above its strike, at the money when the two are close, and out of the money when the underlying trades below it - with puts mirrored in the opposite direction.

Index options like SPX list a wide ladder of strikes at fixed increments around the current price, refreshed with new expirations regularly. Where customers concentrate their buying and selling across that ladder becomes the anchor for dealer hedging obligations at those specific levels - the raw material behind call walls, put walls, and pin risk described elsewhere in this glossary.

Because 0DTE contracts settle the same day they are listed, the strikes immediately around the current price carry a disproportionate share of that session's total gamma and hedging activity - a much narrower, more concentrated ladder effect than in longer-dated options where open interest is spread across a wider range of strikes and expirations.

A strike with heavy open interest is a structural feature worth watching, not a guarantee that price will respect it. The underlying can and does trade through heavily populated strikes; the concentration changes the character of the hedging flow around that level, it does not fix the outcome.

Index option chains typically list strikes at tighter increments near the current price and wider increments further away, giving traders fine granularity close to the money while keeping the total number of listed strikes manageable further out. Exchanges periodically add new strikes as the underlying moves, keeping the ladder centered reasonably close to the current price throughout a listing's life.

Selecting a strike is really selecting a point on the delta spectrum: a strike close to the current price carries a delta near the middle of its range and a relatively even chance of finishing in or out of the money, while a strike far from the current price carries a delta near its extreme and a correspondingly lower or higher likelihood of finishing in the money. This is why delta and strike distance are often discussed together rather than as separate ideas.

Strike selection also interacts with liquidity: strikes closer to the current price and to standard round numbers typically trade with tighter bid-ask spreads and larger size, while strikes far out of the money can trade thinly, with wider spreads that make both entering and exiting a position more costly in percentage terms even when the underlying view is unchanged.