What 0DTE means
0DTE stands for zero days to expiration: options in their final trading day. Some index products, such as SPX, now list an expiration every trading day, so there are always same-day contracts to trade.
Who holds them matters as much as how many there are. A long option always has positive gamma. A dealer's gamma depends on whether dealers bought or sold those contracts, and heavy volume can leave dealers with a small net position.
How gamma behaves on the last day
As expiration gets closer, gamma piles up at the strike nearest the price and drains away from the others. Near the strike, gamma can be at its largest in the final minutes. A strike even 1% away can have almost none.
Example: an index at $500 with 15% implied volatility (simplified Black-Scholes). At the open, the $500 strike has a gamma of about 0.08 and the $505 strike about 0.05. With one hour left, the $500 strike is about 0.22 and the $505 strike about 0.006. With fifteen minutes left, the $500 strike is about 0.43 and the $505 strike is effectively zero. If the price drifts to $505 late in the day, that strike becomes the sensitive one instead.
What the evidence says
Cboe's research found that net market-maker gamma in SPX 0DTE options is small compared with the liquidity of S&P 500 futures, and that customer 0DTE flow is close to balanced between buying and selling (Cboe, 2023 and 2025; Cboe lists these products). An academic study by Dim, Eraker and Vilkov (2024, working paper) found that 0DTE positioning did not amplify index moves in their sample.
Caveat: those are averages over the periods studied. They leave room for individual days when same-day positioning is lopsided. An expected move estimated from today's options is a model estimate for a stated time window, not a guaranteed high-to-low range. Orogex's expected range is one standard deviation, from each expiration's implied volatility; see the Methodology page.