# U.S. Option Symbology

Decode the 21-character OSI option key and learn what the symbol does not tell you.

Canonical: https://wiki.fcontext.com/options/option-symbology/
Fact checked: 2026-07-22

> For educational purposes only; not investment advice.

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## Direct answer

The standard OSI option key identifies a U.S. listed option series with 21 characters: a six-character root, a six-digit expiration date, one call/put character, and an eight-digit strike field. Reading it reduces order-entry mistakes, but the key alone does not establish the multiplier, deliverable, exercise style, settlement method, or last trading time.

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## How the fields work

The fixed-width layout is:

`ROOT__ + YYMMDD + C/P + STRIKE×1000`

The root occupies six positions and is right-padded with spaces. The date uses year, month, and day. `C` means call and `P` means put. The strike is stored as an integer with three implied decimal places, padded to eight digits; divide it by `1,000` to recover the displayed strike.

Spaces matter in machine data even when a screen removes them. A broker may show a compact symbol, a friendly series label, or its own identifier. Treat those as display mappings and verify the readable underlying, exact expiration, side, and strike on the order ticket and confirmation.

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## Worked example

Consider the hypothetical key `AAPL  270115C00150000`:

| Positions | Value | Meaning |
| --- | --- | --- |
| 1–6 | `AAPL··` | root `AAPL`, followed by two spaces |
| 7–12 | `270115` | expiration date January 15, 2027 |
| 13 | `C` | call |
| 14–21 | `00150000` | `150000 ÷ 1,000 = $150.000` strike |

Thus the key describes an AAPL January 15, 2027 `$150` call series. It does **not** prove that one contract delivers 100 ordinary shares. Before trading, inspect the contract specification and any OCC information memo. A corporate action can create an adjusted option with a different root or deliverable even when its strike and expiry look familiar.

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## Verification and risks

- Compare the full key, not only the root or strike; weekly and standard monthly series can differ only by date.
- Preserve leading zeros in the strike field and spaces in the root when processing data.
- Do not infer the economic underlying solely from an unfamiliar adjusted root.
- Confirm multiplier, pricing unit, complete deliverable, exercise style, settlement, expiration procedures, and broker cutoff separately.
- Market-data vendors can use different display aliases; mapping errors can join quotes or positions to the wrong series.

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## Common misconceptions

**“The last eight digits are dollars.”** They are thousandths of a dollar; `00150000` represents `$150.000`, not `$150,000`.

**“Every symbol represents 100 shares.”** Standard equity options commonly use 100 shares, but adjusted contracts can have different deliverables.

**“The symbol contains every contract term.”** It identifies a series; authoritative specifications and OCC notices supply terms the key omits.

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## Related topics

- [Option chain](../option-chain/)
- [Adjusted options](../adjusted-options/)
- [Expiration date](../expiration-date/)
- [Strike price](../strike-price/)

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## Authoritative sources

- [OCC Infomemo 26853: Contract Adjustments and OSI](https://infomemo.theocc.com/infomemos?number=26853)
- [Cboe TOP Specification: 21-character OSI Symbol](https://www.cboe.com/document/tech-spec/document/technical-specifications/cboe-titanium-u.s.-options-complex-multicast-top-specification)
- [Cboe U.S. Options Symbology Reference](https://www.cboe.com/document/tech-spec/document/technical-specifications/cboe-titanium-u.s.-equitiesoptionsfutures-symbology-reference)
- [Nasdaq: Options Defined](https://www.nasdaq.com/articles/options-defined-2019-06-10)