0DTE & SPX options

A quick primer on the instruments 0DTESPX.com simulates. If you already trade SPX options, skim the contract specifics and move on.

What is an option?

An option is a contract giving its holder the right — but not the obligation — to buy (a call) or sell (a put) an underlying at a fixed strike price, up until expiration. The buyer pays a premium for that right; the seller collects the premium and takes on the obligation.

• A call gains value as the underlying rises above the strike.
• A put gains value as the underlying falls below the strike.

Each listed option represents 100 units of the underlying, so a quoted price of 4.30 means 4.30 × 100 = $430 per contract.

A long option's loss is capped at the premium paid, while a short option mirrors the picture: the seller keeps the premium if the option expires worthless, and pays the intrinsic value if it doesn't. That asymmetry is why the platform requires every short to be covered by a long of the same type — see defined-risk only.

What does "0DTE" mean?

0DTE stands for zero days to expiration — options that expire the same day you trade them. SPX lists options expiring every trading day, so on any given session there's a contract that will settle at the 4:00 PM ET close. Because there's no overnight risk and time decay is concentrated into a single session, 0DTE options behave very differently from longer-dated ones:

• Theta is huge and accelerating. An option's entire remaining time value must decay to zero by 4:00 PM ET. Premium that would erode over weeks on a monthly contract erodes in hours — fastest in the final hour.
• Gamma is extreme near the money. Close to expiration, an at-the-money option's delta flips quickly as SPX crosses the strike, so position P&L can swing sharply on small index moves.
• Every day is a complete round trip. A position opened at 10:00 AM is guaranteed to be resolved — closed, expired, or cash-settled — by the close. There is nothing to carry, and nothing to wake up to.

That compressed lifecycle is what makes 0DTE both popular and unforgiving — and why it rewards exactly the kind of systematic testing this platform is built for.

Why SPX?

SPX options track the S&P 500 index. They have three properties that make them popular for 0DTE trading:

• Cash settlement — there are no shares to deliver. At expiration an in-the-money option simply settles for cash equal to (underlying − strike) × 100. You never get assigned 100 shares of anything.
• European exercise — SPX options can only be exercised at expiration, not before, so there's no early-assignment risk.
• Deep liquidity across a wide range of strikes, every trading day.

The simulator also recognizes the related cash-settled index products NDX, VIX, and XSP for settlement purposes.

Moneyness

"Moneyness" describes where the strike sits relative to the underlying:

• In the money (ITM) — a call with strike below the underlying, or a put with strike above it. Has intrinsic value.
• At the money (ATM) — strike nearest the underlying.
• Out of the money (OTM) — a call above the underlying, or a put below it. All premium, no intrinsic value.

An option's premium splits into intrinsic value (what it would be worth if it expired right now) and extrinsic value (everything above that — pure time value). For a 0DTE option, extrinsic value is precisely the part that will be gone by 4:00 PM ET; selling it is the basis of most income-style 0DTE strategies, and buying it is a bet that a move will outrun the decay.

SPX contract specifics

Property	Value
Underlying	S&P 500 index (SPX)
Settlement	Cash, (underlying − strike) × 100
Exercise style	European (at expiration only)
0DTE expiry	4:00 PM ET on the trading day
Contract multiplier	100
Strike grid	Every 5 points around the money

In the simulator, an SPX option is identified by its root, expiry date, side, and strike — for example an SPX 5950 call expiring at the day's 4:00 PM close. The API writes this as a canonical OPRA/OSI instrument string like SPXW 250115C05950000 (the SPXW weekly root, the YYMMDD expiry date, the side letter, and the strike × 1000).

A note on risk

0DTESPX.com is a paper-trading simulator for education and research. Nothing here is investment advice, and simulated results don't guarantee real-world outcomes. Options trading carries substantial risk of loss. Next: see how trading is organized in Accounts & trading days.