# Strike concentration and pinning

**DL-06 · Dealer Logic · Core**

**Lesson question:** Why does large open interest not guarantee a pin?

Original fictional examples; educational writing, not live market data, orders or production approval.

## By the end

- Explain the mechanism in the lesson question.
- Reproduce the worked comparison and recognize its failure case.

**Prerequisites:** DL-01

A large option concentration can make a strike relevant to hedging, particularly near expiration. It does not force settlement at that strike. Open interest counts contracts, each with a long and a short, but does not by itself establish dealer ownership.

## Understand the mechanism

At-the-money Gamma can concentrate as expiry approaches, while far-away options behave differently. A positively signed, actively hedged inventory may generate opposing flow around a strike; a negatively signed inventory may amplify movement.

Other expiries, settlement mechanics and external orders can outweigh that channel. A concentration is a research location, not a guaranteed magnet.

## Walk through the process

Identify the exact product, expiry and sensitivity measure. Separate unsigned OI from signed exposure and compare possible hedge size with available liquidity.

Keep a move-away branch in the plan. Holding a short option into expiry solely because of a presumed pin leaves assignment and gap risk unresolved.

## Evidence boundary

The arithmetic is original conditional teaching material, not a measurement of current dealer positions. The cited Cboe research is historical product-specific evidence, not a live inventory feed. Model outputs should remain distinguishable from observed market facts.

## A worked example

A fictional strike has 1,000 calls open and per-unit Gamma 0.04. With a 100 multiplier, the magnitude is 4,000 shares per dollar. A dealer long all of them has +4,000; a dealer short all has −4,000.

The same unsigned OI supports opposite possible hedge directions. The chart makes that missing sign explicit rather than inferring it.

## Graph: Strike concentration and pinning—worked comparison

**Horizontal:** Ownership assumption  
**Vertical:** Gamma (shares/USD)

Fictional data from the written example. The chart organizes the comparison; its shape is not a forecast.

Complete numerical data are embedded in `scripts/dealer-logic/DL-06.js` and displayed as an accessible table in the HTML lesson.

## What would invalidate the reading?

Assuming that all open calls are dealer-long calls can invert the entire explanation. Strong external flow can also overwhelm genuine pinning feedback.

## Check your understanding

### Does 1,000 contracts of OI establish dealer Gamma's sign?

**Answer:** No.

**Why:** Ownership and offsetting positions must be known or explicitly modeled.

### What is the critical failure to avoid?

**Answer:** Assuming that all open calls are dealer-long calls can invert the entire explanation. Strong external flow can also overwhelm genuine pinning feedback.

**Why:** Explain this risk before treating the example as a decision rule. A correct calculation does not guarantee a favorable market outcome. **Critical risk check.**

## Takeaway

Return to the question: Why does large open interest not guarantee a pin? State the observed facts, the assumptions and the failure condition separately.

## References and provenance

The cited references support definitions and conventions. The teaching prose, tickets and numerical scenarios are original; they are not attributed as historical market observations.

- [OIC Greeks](https://www.optionseducation.org/advancedconcepts/understanding-options-greeks)

- [Cboe 0DTE positioning study, May 2025](https://www.cboe.com/insights/posts/0-dt-es-decoded-positioning-trends-and-market-impact)