S&P 100 (OEX)

The S&P 100, commonly identified by the price-index ticker OEX, is a float-adjusted-market-capitalization-weighted index of 100 major companies selected from the S&P 500. S&P Dow Jones Indices generally selects large S&P 500 companies whose individual stocks have listed options and also considers sector balance. “OEX” can additionally refer to Cboe’s American-style, cash-settled options on the index, so the index and the option contract should not be treated as the same instrument.

Key Takeaways

• The S&P 100 is a selected subset of the S&P 500, not an automatic ranking of its 100 largest stocks.
• Size, listed options, and sector balance influence selection; the Index Committee has discretion.
• Constituents are weighted by float-adjusted market capitalization.
• One company can contribute more than one listed share class, so a factsheet can show more securities than the 100-company target.
• S&P 100 membership changes on an as-needed basis rather than through a scheduled annual reconstitution.
• OEX is used for the price index and for Cboe’s American-style S&P 100 index options.
• Price, total-return, and net-total-return series are different benchmarks.

What the S&P 100 Measures

The S&P 100 measures the performance of a concentrated group of established U.S. large-cap companies. Its selection universe is the S&P 500. It therefore inherits the parent index’s basic requirements, including U.S. domicile for index purposes, eligible U.S. exchange listing, security-type rules, liquidity, investable float, and financial viability for additions.

The Index Committee then selects 100 companies. The current methodology says it generally favors the largest S&P 500 companies that have listed options while considering sector balance. There is no published rule requiring the index to hold the exact 100 largest companies by market capitalization.

The company target and security-line count can differ. If multiple eligible public share classes of one company are represented separately, the index may contain more than 100 securities while still representing 100 companies.

How Constituents Are Weighted

The index uses an Investable Weight Factor (IWF) to exclude strategic or otherwise non-public holdings from the weight calculation. A simplified float-adjusted market value is:

The simplified constituent weight is:

where (P_i) is price, (S_i) is the index share count, and (IWF_i) is the investable weight factor. The index level uses a divisor:

The divisor is adjusted for constituent changes and qualifying corporate actions so that mechanical changes do not create false index returns.

Worked Weighting Example

Assume a simplified index with three security lines:

If A gains 2%, B loses 1%, and C gains 4%, the simplified price return is:

The example excludes dividends, corporate actions, share updates, and intraperiod weight changes. It shows why the largest float-adjusted companies can dominate index performance.

Selection and Maintenance

S&P 100 constituent selection is at the Index Committee’s discretion. Because candidates already belong to the S&P 500, the S&P 100 is not independently rebuilt from all U.S. stocks.

S&P’s U.S. indices methodology states that S&P Composite 1500 membership changes, including S&P 500 changes, occur as needed in response to corporate actions and market developments. S&P 100 membership likewise has no scheduled reconstitution described in its construction rules. Share counts are updated quarterly, and material IWF changes are handled under the provider’s maintenance policies.

This distinction matters:

• constituent change: a company enters or leaves the index;
• share or IWF update: an existing constituent’s weight changes without changing membership;
• market movement: a weight changes because its share price moves relative to others.

Analysts should use the provider’s dated constituent files and announcements rather than assume a quarterly weight update means all companies were reselected.

S&P 100 vs. S&P 500 and Nasdaq-100

The S&P 100 is more concentrated than the S&P 500. Nasdaq-100 is not a substitute: it uses a different listing universe, excludes financial companies, and follows a different weighting methodology.

Price and Total-Return Series

For a U.S. index, the net and gross series may sometimes be identical for certain investor assumptions and periods, but they remain separately defined series. Always match the exact ticker, currency, and return convention used by the portfolio or product.

OEX Index vs. OEX Options

The index is a calculated benchmark. An OEX option is a derivative contract whose payoff depends on the S&P 100 index level.

American-style exercise means an OEX option can generally be exercised on a business day through expiration. Cboe separately lists XEO options on the S&P 100 with European-style exercise. Contract specifications can change, so the current exchange page and broker documentation control.

Worked OEX Option Example

Assume an OEX call has a strike of 3,000, the exercise-settlement value is 3,025, and the contract multiplier is USD 100. Its settlement amount before the premium and transaction costs is:

If the buyer paid a premium of 18 index points, the premium was USD 1,800 before fees. The illustrated net result before taxes and fees would be USD 700:

If the settlement value were at or below 3,000, the call’s exercise value would be zero and the buyer could lose the premium paid. An uncovered option writer can face much larger losses. This example is educational and omits margin, early exercise, bid-ask spreads, commissions, and tax treatment.

Why the S&P 100 Matters

• Large-cap benchmark: it provides a concentrated reference for major U.S. companies.
• Derivatives underlying: listed options allow market participants to hedge or express views on the index, subject to substantial risk.
• Performance attribution: analysts can compare a concentrated blue-chip portfolio with the broader S&P 500.
• Market context: the index summarizes movements among highly influential U.S. companies.

The index is not a complete measure of the U.S. economy. It omits smaller public companies and private businesses, and many constituents earn substantial revenue outside the United States.

Risks and Limitations

• Concentration risk: 100 companies provide less breadth than the S&P 500, and the largest weights can dominate returns.
• Sector risk: considering sector balance does not produce equal sector weights.
• Selection discretion: membership is not generated by a fully mechanical top-100 ranking.
• Revenue geography: U.S. domicile does not imply that revenue or operations are primarily domestic.
• Return-series risk: price, total-return, and net-total-return series are not interchangeable.
• Product risk: funds add fees and tracking differences; options add leverage, time decay, volatility, liquidity, exercise, settlement, and potentially large-loss risks.
• Specification risk: exchange option terms and index methodology can change.

How to Evaluate OEX Data or Products

1. Determine whether “OEX” refers to the price index, an option, or a data-vendor symbol.
2. For index analysis, identify the price, total-return, or net-total-return series and dates.
3. Review top-company and sector concentration.
4. Distinguish 100 companies from the number of represented security lines.
5. For a fund, verify its benchmark, fees, tax treatment, and tracking difference.
6. For an option, verify exercise style, multiplier, settlement method, expiration, liquidity, and margin with current documents.
7. Do not infer suitability or expected returns from index inclusion.

Common Mistakes

• Calling the S&P 100 an automatic list of the 100 largest U.S. stocks.
• Treating the S&P 100 as equally weighted or weighted by full shares outstanding.
• Assuming a quarterly share update is a quarterly constituent reconstitution.
• Confusing the OEX index ticker with the OEX option contract.
• Assuming all S&P 100 options have the same exercise style as OEX.
• Comparing the OEX price index with a dividend-reinvesting portfolio.
• Describing a cash-settled index option as delivering a basket of 100 stocks.
• Treating index or option exposure as a personalized investment recommendation.

Authoritative Sources

• S&P Dow Jones Indices: S&P 100 provides the official objective, current characteristics, tickers, constituents, and linked products.
• S&P U.S. Indices Methodology defines eligibility, S&P 100 selection, float-adjusted weighting, maintenance, and return series.
• S&P Index Mathematics Methodology explains float-adjusted capitalization and divisor calculations.
• Cboe: OEX Options Product Specifications provides the current contract multiplier, exercise style, expiration, and settlement terms.

Related Terms

• S&P 500: The parent index from which S&P 100 companies are selected.
• Float-Adjusted Market Capitalization: The investable-value approach used for constituent weights.
• Benchmark Index: A rules-based reference used to assess a portfolio or mandate.
• Option: A derivative contract granting its holder a conditional exercise right.
• Index Fund: An investable vehicle that seeks to track a benchmark before costs and tracking differences.
• Total Return: Performance combining price changes and distributions.

FAQs

This article is educational and does not provide investment, tax, or legal advice. Options involve substantial risk and are not suitable for every investor.