OpEx Week Gamma:
Why Expiration Week Is Different
- OpEx week = the week containing the third Friday of the month, when monthly options expire with the largest accumulated OI.
- Monthly options carry 3–5× the OI of a standard weekly chain — creating the biggest gamma event of the month.
- Price gravitates toward the pin strike as Charm decay accelerates and dealers defend their gamma positions.
- Realized volatility is often suppressed during OpEx week, then expands the following week when the suppressing gamma expires.
- The day after monthly expiration is structurally one of the most likely times to see a volatility regime change.
What Makes OpEx Week Special
Every week in the options market is an expiration week for some chain. SPY has daily expirations (0DTE), weekly expirations (every Friday), and monthly expirations (third Friday of the month). But not all expirations are equal. Monthly expiration carries the largest, most heavily populated options contracts — and that changes everything about how the market behaves structurally.
The reason comes down to open interest concentration. Monthly options build up OI for weeks or months. By the time OpEx week arrives, the monthly chain typically carries 3–5× the OI of a standard weekly chain. That means 3–5× the gamma exposure, 3–5× the Charm decay, and 3–5× the dealer hedging obligations. The structural forces are just bigger.
Monthly Options vs Weekly Chains
| Characteristic | Standard Weekly | Monthly OpEx |
|---|---|---|
| Open Interest | Moderate | Very high (3–5× weekly) |
| Gamma near ATM | Elevated into Friday | Extremely elevated Thursday/Friday |
| Charm decay rate | Normal | Accelerated; faster near ATM strikes |
| Pin tendency | Moderate | Strong; dealers defend large OI strikes |
| Post-expiry vol reset | Minor | Significant; large gamma position expires |
Gamma Concentration at Expiry
Gamma spikes dramatically as expiration approaches for at-the-money options. An ATM option with 30 days to expiry might have a gamma of 0.03. The same ATM option with 1 day to expiry might have a gamma of 0.30 or higher — a 10× increase in hedging sensitivity.
Multiply that by the massive open interest in monthly options. Dealers at ATM strikes are making extremely large hedging adjustments for very small price moves. This creates a self-reinforcing dynamic: price moves toward a high-OI strike → gamma spikes → dealers counter-hedge aggressively → that counter-hedging pulls price back. The strike becomes a gravitational anchor.
OpEx Pinning: How and Why It Happens
OpEx pinning is the tendency for the underlying to close near the strike with the most open interest on expiration day. It's the mechanical result of dealers defending their largest gamma positions.
Say SPY is at 510 on OpEx Thursday with massive OI at the 510 strike:
- If SPY moves to 511: Dealers with short 510 calls have too much delta → they sell underlying → SPY gets pushed back toward 510.
- If SPY moves to 509: Dealers with short 510 puts have too much short delta → they buy underlying → SPY gets pulled back toward 510.
Both extremes trigger counter-forces. The pin self-reinforces as long as open interest is large relative to the directional volume trying to break it.
Volatility Suppression During OpEx
The same dealer hedging that creates pinning also suppresses realized volatility during OpEx week. When dealers are aggressively counter-hedging near the pin strike, they mechanically dampen intraday ranges. The result is recognizable: tighter daily ranges, fewer sustained intraday trends, more mean-reversion. Breakout strategies underperform. Range-bound strategies outperform.
Implied volatility often drops during OpEx week as well, as the upcoming expiration removes near-term uncertainty. Declining IV further reduces Vanna flows, compounding the range-bound tendency.
Post-OpEx: The Gamma Reset
When monthly options expire Friday afternoon, a massive gamma position simply disappears. The Call Walls and Put Walls that anchored price all week no longer exist. The structural suppression of volatility evaporates overnight.
The following Monday through Wednesday is structurally prone to higher realized volatility:
- Gamma vacuum: Without the large monthly OI, the next-largest expiry carries far less open interest. Price can move more freely.
- Re-positioning flows: Traders rolling to new strikes create new OI at different levels, which takes time to accumulate structural mass.
- Vanna tailwinds may shift: If IV was suppressed during OpEx week, the post-expiry IV repricing can create Vanna flows in either direction.
Triple & Quad Witching
Four times per year (March, June, September, December), monthly options expiration coincides with the expiration of index futures, equity futures, and single-stock futures — known as "quad witching." The total notional value is dramatically higher than a standard monthly OpEx. Pinning, vol suppression, and the post-expiry reset are all correspondingly more intense. The Friday of quad witching is one of the highest-volume trading days of the year.
Reading GEX During OpEx Week
During OpEx week, the GEX chart has characteristic features:
- Taller, sharper peaks: The monthly chain contributes a dominant spike at the main OI strikes. This spike is your pin candidate for expiration Friday.
- Rapidly rising net GEX: As days pass toward expiration, ATM gamma spikes and net GEX often reaches its monthly peak on Thursday/Friday.
- Call Wall = likely pin: The highest positive GEX spike at a round number near ATM is typically the gravitational center for OpEx Friday's close.
GEXBoard updates every 15 minutes throughout the session, allowing you to track how the GEX landscape shifts as expiration approaches. When the highest-GEX strike is near current price at a round number, the pin probability is highest.
Identify the OpEx pin strike live
GEXBoard shows the Call Wall, Put Wall, and full GEX profile updated every 15 minutes — giving you a live view of dealer gamma positioning through OpEx Friday. From $9/mo during beta.
Frequently Asked Questions
What is OpEx week?
OpEx week is the week containing the third Friday of each month, when monthly options contracts expire. Monthly options carry far more open interest than weekly chains, making their expiration the largest structural gamma event of the month.
Why does price pin near a strike during OpEx?
Dealers with large gamma positions at high-OI strikes hedge aggressively in both directions when price approaches those strikes. If price moves above, they sell. If it moves below, they buy. This counter-hedging creates a gravitational pull — especially in the final hours before expiration when gamma is at its peak.
What happens to volatility after monthly OpEx?
The large gamma positions that suppressed volatility during OpEx week expire. Without those positions, dealers have less counter-trend hedging obligation and price can move more freely. The first few days after monthly expiration typically see higher realized volatility as the gamma vacuum is filled by new positioning.
How is quad witching different from regular monthly OpEx?
Quad witching (March, June, September, December) adds index futures, equity futures, and single-stock futures expiration on top of monthly options. The total notional value is dramatically higher, amplifying all the same gamma dynamics: tighter pin, stronger suppression during the week, and a more pronounced volatility reset afterward.
How can I identify the pin strike using GEXBoard?
The pin strike is typically the highest GEX spike on the chart nearest to the current price, especially at a round number. During OpEx week, look for the Call Wall (highest positive GEX concentration) in the near-ATM zone — that strike is the most likely gravitational center for expiration Friday's close.