Same-Day SPX Is Now the Main Event
Zero-days-to-expiry trading is no longer a niche corner of the S&P 500 options market, it is most of it. The exchange's Q3 2025 State of the Options Industry report put SPX 0DTE average daily volume at 2.15 million contracts, or 57% of all SPX index options volume. The exchange's own strategy coverage adds that roughly half of that same-day activity comes from retail traders.
That scale changes what a "0DTE SPX strategy" needs to be. Every structure in the playbook (iron condor, credit spread, butterfly, long vertical) is public knowledge and available to everyone at the same prices. Lists of strategies are a commodity, and most guides stop there. What they leave out is the selection step: reading the day's dealer positioning and how each structure behaves under it. Our zero DTE options guide covers the raw mechanics of same-day options, extreme gamma, brutal theta, binary payoffs. This article covers the decision layer that sits on top.
Why SPX Is the Default 0DTE Vehicle
SPX options settle in cash, so nothing is assigned or delivered at expiration, a losing position resolves to a debit, not a surprise share position. They are European-style, which removes early-assignment risk entirely: no leg of a spread can be exercised against you before the close. Both properties matter far more at zero days to expiry than at thirty, because 0DTE structures routinely expire with the index sitting on or near a short strike. The full comparison, including contract size and liquidity trade-offs, is in our SPX vs SPY options guide.
There is also a tax dimension: as cash-settled broad-based index options, SPX contracts fall under Section 1256 of the tax code, which applies a blended 60% long-term / 40% short-term capital gains treatment regardless of holding period. How that nets out depends on your situation, that is a question for a tax professional, not a trading article, but it is part of why active same-day traders concentrate in SPX rather than equity ETF options.
Read the Gamma Regime Before Picking a Structure
Here is the framework most strategy roundups skip. On any given day, dealers are net long or net short gamma in SPX, and that positioning shapes how the index moves intraday.
In positive gamma conditions, dealers hedge by selling into rallies and buying into dips. Their flow leans against price movement, which tends to compress ranges and favor mean reversion. That is the condition short-premium structures depend on: their credit is kept only if the index stays inside a range.
In negative gamma conditions, the hedging flips: dealers sell weakness and buy strength, amplifying whatever move is underway. Those conditions are associated with trend days, gap extensions, and air pockets, the setting in which a "safe" iron condor sold at 10 AM can be a full loss by 2 PM.
The boundary between the two regimes is the gamma flip level, and where the index sits relative to it is the piece of positioning information this framework turns on. If the mechanics are new to you, start with what gamma exposure is and how the flip price works.
Above the Flip: the Premium Seller's Day
Positive gamma days are when defined-risk premium selling has structural tailwind. The canonical structure is the same-day iron condor, and the exchange's deep dive on Henry Schwartz's zero-day SPX iron condor lays out the logic cleanly: sell an out-of-the-money put spread and an out-of-the-money call spread, each built for roughly 9:1 risk-to-reward. Because the index cannot finish both above the call strikes and below the put strikes, at most one side can lose, you are effectively making two high-probability trades where only one can be wrong. The same coverage also notes the entry timing that tends to accompany the approach: after intraday volatility spikes, such as the reaction to a morning economic release, rather than during quiet consolidation. One reading of that timing is that premium is briefly rich after a spike and has already drained during a lull.
Be honest about what 9:1 means, though. Each spread that risks nine to make one needs to win roughly nine times in ten just to break even before costs, and more than that after them. The full condor's math is slightly friendlier, since only one side can lose, but the shape is the same. That is the trade-off every 0DTE premium seller accepts: a high win rate with a tail loss that erases weeks of gains when the regime is misread. The framework adds two conditions. The first is the regime: the structure assumes positive gamma is actually dampening movement. The second is strike placement relative to where dealer hedging concentrates: the day's call wall and put wall mark the strikes where opposing flow is estimated to be thickest, and short strikes placed beyond them sit on the far side of that flow.
Below the Flip: Trend-Day Tactics
When the index is trading below the flip, the premium seller's math inverts. Negative gamma means dealer hedging accelerates moves, ranges expand, and the tail outcomes that a 9:1 structure cannot afford become materially more likely. In this regime the framework turns to directional, defined-risk structures: a debit vertical in the direction of the move risks a known premium for a capped payout.
A naked 0DTE call or put bought at the open pays for implied volatility and premium at their richest, and theta grinds the position down during any midday pause. A vertical spread blunts that decay by financing the long strike with a short one further out. Selling premium against a negative-gamma trend because the index "has gone too far" runs against the hedging flow. Structurally, negative gamma days do not owe anyone mean reversion; the hedging flow is pushing with the trend, not against it.
The Clock Is Part of the Trade
The same structure behaves like three different trades depending on when it is opened. In the first half hour, premium is richest and direction is noisiest, attractive for sellers, dangerous for buyers. Through midday, theta does most of the work on short-premium structures while directional positions stagnate. In the final hour, gamma reaches its extreme: near-the-money 0DTE deltas swing violently with small index moves, and a position that was comfortably out-of-the-money at 3 PM can be at-the-money at 3:50.
That late-day behavior is the reason given for closing winning same-day premium positions before the final stretch rather than holding them through the settlement print. The last hour is also when pin risk around heavily-traded strikes is at its strongest, with price oscillating around large open interest as hedges unwind. Where that open interest is heaviest for the day's expiration is on the free SPX max pain page. Holding for the last few cents of a credit means carrying binary risk through the most convex hour of the session.
Sizing Is the Strategy
Structure selection is one decision; sizing decides how much any single misread day can cost. The sizing rules discussed for same-day trading are unglamorous: a per-trade loss cap set as a fixed fraction of the account, defined-risk structures only (spreads, condors, butterflies) so the worst case is known at entry, and no adding to a losing 0DTE position, since with hours to expiry there is no "long term" for the trade to recover into. Each day is also an independent event: a strategy that won four days straight says nothing about whether today is a positive or negative gamma day.
A Pre-Open Checklist
A repeatable 0DTE SPX process fits in five questions. Where is the index relative to the gamma flip, and how far away is it? Where are the call wall and put wall sitting? Is there a scheduled catalyst (CPI, FOMC, a Treasury auction) that could flip the regime mid-session? Given the answers, how does each structure described above behave in today's regime? And finally: at the chosen size, what is the worst case as a share of the account?
SquawkFlow's GEX page publishes the flip level and walls computed from daily-settled open interest. It includes 0DTE positions carried through the prior settlement, not contracts opened and closed during the current session.
Educational content, not financial advice. See our risk disclosure.