Summary. Same-day SPX options, known as 0DTE, have hours of life rather than weeks, and their gamma is more concentrated and shorter-lived than that of any other expiry. Cboe has reported that same-day contracts make up more than half of all SPX options volume, so they are a central part of the intraday dealer book. This guide explains how 0DTE gamma differs from longer-dated gamma, how its profile changes through the session, how same-day walls form and fail, and when 0DTE hedging amplifies moves instead of damping them.
1. What makes 0DTE different
Since Cboe added Tuesday and Thursday expirations to SPX Weeklys in 2022, an SPX option expires on every trading day, and Cboe has reported monthly records above 60% of SPX volume for same-day contracts. A standard listed option has weeks or months of time value; a 0DTE option has hours. Two properties follow.
Gamma is very large near the money. For an at-the-money option, Black-Scholes gamma is approximately
so at the same strike and implied volatility it scales with one over the square root of time to expiry. Counting time in trading hours, with one 6.5-hour session as one trading day, an at-the-money option with six hours left has about times the gamma of one with six sessions left, and with one hour left about times.
That gamma disappears within the session. The width of the profile is set by the expected move to expiry, . For a hypothetical SPX at 6,000 with 16% implied volatility, one standard deviation to the close is about 60 points at the open and about 24 points with an hour left, and a strike three standard deviations away carries roughly 1% of the at-the-money gamma. At expiry delta becomes a step at the strike, and the same-day hedging profile collapses onto the few strikes nearest the closing price.
2. Why the morning differs from the afternoon
In the morning, same-day gamma is spread across a wide band of strikes, each with a modest share, and overnight positioning, longer-dated expiries and macro flow tend to dominate. Whether same-day hedging damps or amplifies moves at this stage depends on the sign of dealer positions, not on the time of day.
As the session goes on, the band narrows and the peak rises, right up to the close. In the last hour gamma sits in a few strikes around spot and the dealer response to a move through them is at its sharpest, which is why the final hour is where same-day pinning and same-day accelerations are both most often seen.
A map built only from the morning's open interest misses much of this. Open interest is reported once a day, and many same-day positions are opened and closed within the session, so a morning snapshot understates positioning built later. Estimates that assign the day's volume by the side that initiated each trade fill part of the gap, with more noise.
3. 0DTE call walls and put walls
Walls in the same-day expiry form and dissolve within hours. Same-day positions tend to gather at round-number strikes near spot, and because gamma there is so concentrated late in the day, the dealer response when price arrives is strong. Its direction depends on who holds which side:
- Dealers long gamma at the strike, for example after customers sold calls above spot or puts below it. Hedging sells rallies toward the strike and buys declines toward it, so the strike resists and can pin. This is what is usually meant by a wall.
- Dealers short gamma at the strike, for example after heavy customer call buying above spot. As price approaches, dealer hedging buys, which can carry price into and through the strike. The same holds on the downside where customers bought puts.
The usual sign convention counts call open interest as dealer-long, so a same-day call wall built on customer call buying can speed price up rather than hold it back. Filtering the map to the same-day expiry separates a wall that will matter by the afternoon from one that belongs to next month. GEX levels explained covers how walls are defined.
4. When 0DTE adds volatility instead of compressing it
A common belief is that same-day options always pin price. They do not. When customers are net buyers of same-day options near spot, dealers are short gamma there, and their hedging adds to moves. A falling market with heavy customer buying of same-day puts is the clearest case: as the index drops toward those strikes, dealers sell to stay hedged, and when that meets an outside catalyst the intraday range can widen sharply.
The aggregate evidence is more balanced than either story. Cboe describes same-day SPX flow as fairly balanced between retail and institutional traders, and customer demand runs both ways, so dealer gamma in the same-day expiry is not uniformly negative. Dim, Eraker and Vilkov find that high 0DTE gamma does not carry past volatility forward and that intraday 0DTE volume shocks do not amplify recent index returns. The sign of dealer positioning, which varies from session to session, decides whether the effect near a large strike is a pin or an acceleration. Price alone cannot tell which; it takes a same-day gamma estimate, read knowing its sign convention is weakest where customer buying is heaviest.
5. Practical takeaways
- Re-check the map during the session. The same-day profile changes faster than any other expiry, and faster still after midday.
- Watch the same-day walls. The front-expiry call wall and put wall are where same-day gamma concentrates, and where an acceleration or a pin is likely to start.
- Treat pinning stories with caution when same-day gamma is negative. A pin needs dealers long gamma at the strike; short gamma pushes price away from strikes instead.
- Expect the extremes in the last half hour. Same-day gamma and charm both peak, so ranges can be very narrow or very wide depending on the sign of dealer gamma at the dominant strike. Risk set for midday conditions may not fit the close.
The DataFirst terminal shows these levels. How gamma affects SPX follows the whole session from open to close.
For educational and informational purposes only. Not financial advice. Options trading involves substantial risk of loss. Model figures use Black-Scholes with a flat, hypothetical implied volatility; real same-day volatility surfaces have skew and curvature.