Education · 0DTE Gamma

0DTE Gamma: Why Same-Day Options Move SPX

Same-day-expiry SPX options — universally known as 0DTE — have grown from a fringe product to roughly half of all SPX options volume in just three years. They are the most mechanically loud part of the dealer book, and learning how their gamma builds, decays, and accelerates is now a basic literacy requirement for any active SPX trader.

What makes 0DTE different

A standard listed option has weeks or months of time value. A 0DTE option has hours. Two things follow from that:

Gamma is enormous near the money. The gamma of an at-the-money option scales inversely with the square root of time to expiry — so an option with 6 hours left has roughly 5x the per-contract gamma of one with 6 trading days left at the same strike.
That gamma evaporates within the same session. By 15:00 ET on expiration day, options more than a few dollars out-of-the-money have negligible gamma. The whole hedging profile collapses into the strike where price actually closes.

The combination of those two facts is why intraday SPX behavior on heavy 0DTE volume sessions is qualitatively different from earlier-decade SPX trading.

Why the morning is different from the afternoon

In the morning, 0DTE gamma is large and spread across many strikes — the options have value at strikes far from spot, so dealer hedging tends to dampen moves. By afternoon, gamma collapses into a narrow band around spot, and the dealer's hedge response gets concentrated. The 14:30–15:30 ET hour is often where you see the sharpest intraday accelerations or pinning, because that is where the 0DTE gamma profile is most peaked and the dealer has the least time to amortize hedges.

Traders who only look at a morning GEX snapshot miss the most important part of the session.

0DTE call walls and put walls

Walls inside a 0DTE expiry build and dissolve within hours. Public flow into 0DTE calls clusters at round strikes 0.5–1.5% above spot in the morning; once price tests those strikes, dealer hedging tends to push back hard, because the gamma at that strike is so concentrated. The same dynamic plays in reverse for 0DTE puts on the downside.

The GEXRadar 0DTE expiration filter lets you isolate this profile from longer-dated expiries. The Heatmap view often shows a "hot stripe" right at the front-month 0DTE column on expiration days.

When 0DTE causes volatility instead of compressing it

A common misconception is that 0DTE always pins price. It does not. When 0DTE positioning is net short by dealers (heavy public put-buying into a falling market, for instance), 0DTE gamma flips negative. In that regime, dealer hedging on the 0DTE strip amplifies the move, and the intraday range can blow out. The August 5, 2024 SPX gap-down and several FOMC days in 2023 share this signature: negative 0DTE gamma meeting an exogenous catalyst.

You cannot tell whether 0DTE is stabilizing or destabilizing from price alone. You need the live GEX snapshot for the front expiry.

Practical takeaways

Refresh your gamma view at least every 15 minutes during a 0DTE session. The map changes faster than at any other expiry.
Watch the front-expiry call wall and put wall specifically — those are 0DTE-dominated and are where the acceleration lives.
Be skeptical of "pinning" narratives when 0DTE GEX is negative. Pinning requires positive dealer gamma; negative-gamma 0DTE pins nothing.
The closing 30 minutes is where 0DTE charm and gamma both peak. Use tighter stops if you're holding into the bell.