0DTE Options: Why Expiry Day Trades Differently

0DTE means zero days to expiry: an option that expires today.

Quick refresher. The strike is the price the option is written at. Delta is how much the option moves when the index moves one point, and it is also the hedge: the dealer, the market maker who took the other side, holds futures to cancel that delta. Gamma is how fast delta changes as price moves. In the money means the option would pay something if it expired now. Out of the money means it would not. Implied vol is the size of move the options market is pricing in, as a yearly percentage. SPX is the S&P 500 index.

FactIndex options now list an expiry for every trading day, so there are always options expiring today. Everything that happens slowly to a 30-day option happens fast to a 0DTE one:

• The delta curve turns into a cliff. Near the strike, delta can swing from almost 0 to almost 1 on a small move, so the hedge has to be traded constantly.
• The range shrinks by the hour. The move the options price in scales with the square root of the time left. By the close it is a fraction of the open.
• Time alone moves delta. That is charm. It forces re-hedging even when price does not move.

Two dates people merge: 2017 is when SqueezeMetrics published the gamma exposure (GEX) paper, the one that popularised estimating dealer hedging from options positions. That is a date about an idea. Around 2022 to 2023, after SPX got an expiry every trading day, 0DTE grew to roughly half of SPX options volume, a date about volume. Figures differ by source, so treat any single number as approximate.

Fact marks Black-Scholes maths and hedging arithmetic. Naive model marks a story about what dealers or price will do: useful, not a fact.

A same-day call on the 6,000 strike. Run the session forward and watch the delta curve stiffen into a step, and the gamma bell below it spike at the strike.

FactThe range the options price in is roughly price x implied vol x the square root of the time left, with time counted in years (6.5 hours is 6.5 / 8,760 of a year). As the hours drain, the funnel closes. Half the session gone leaves about 71% of the move priced at the open, not half of it.

Naive modelOne observation traders use: at the open, mark the strikes near 20 to 25 delta on each side as the edges of the implied move. Through the day that band squeezes in toward price. On the chart they sit inside the one-expected-move band: a 25 delta strike is only about two thirds of one move out. Treat them as a rough marker, not the same line.

FactAs expiry nears, an option commits. An in-the-money call's delta climbs toward 1, an out-of-the-money call's delta fades toward 0, and at the strike it stays near 0.50. That drift comes from time alone, so a dealer's hedge goes out of line on a completely flat tape. Price is frozen at 6,000 here. Only the clock runs.

On expiry day I treat the options map as something that changes by the hour. A strike far from price loses its hedge as the day goes on, and the bar on a positioning screen does not shrink when that happens, because the position is still open. It is the force that left, not the position.

FactA strike right on top of price goes the other way: gamma concentrates there as time runs out, so each point of movement there forces more hedging into the close, not less. Far strikes lose their hedge to charm, near strikes gain hedging per point from gamma: two different reasons.

Naive modelIf price sits inside a level for several minutes without reacting, I stop treating it as a level, whatever the board still shows. And in the last hour, I expect the close to be pulled around by the biggest strikes near price rather than by the morning's levels.

Reading expiry day by the hour is one piece of reading the market. The Mentorship is where you put the whole process together.