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.

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 (time decay) is large and accelerates into the close, and prices are highly sensitive to intraday moves in the underlying.

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.

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

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 a trading day is recreated in Simulations.