Monthly options expiration — OPEX — is the third Friday of each month, when a huge block of options settles and rolls off the board. Because dealer positioning is built on that open interest, the days into and out of OPEX have a rhythm worth knowing.
The pin effect
Into expiration, the largest open-interest strikes exert a stronger and stronger pull. As those options approach expiry, the hedging around them tightens, and price often gravitates toward — and sticks near — a big strike. That's the classic OPEX “pin”: a market that seems glued to a round number into a Friday close.
Vanna and charm build into it
Two second-order flows get louder through OPEX week. Charm — the drift of hedges as time runs out — and vanna — how hedges shift as volatility moves — can quietly tug price toward big strikes into expiration, especially in a calm tape. They're rarely the whole story, but into OPEX they're a bigger part of it.
The market is often most pinned right before it's most free — the calm into expiration, then the reset out of it.
The post-OPEX reset
When expiration passes, a large slice of open interest — and the dealer gamma tied to it — simply disappears. Positioning resets. That's why the days after monthly OPEX can trade looser and more directional than the pinned days before it: the hedging that was dampening moves has rolled off, and the market has to re-establish where the new walls sit.
Triple witching
Four times a year — March, June, September, December — stock options, index options, and futures expire together (“triple witching”). These are the heaviest expirations of all: bigger pins into them, bigger resets out of them, and unusually large closing volume on the day.
The week's rhythm
Put it together and the OPEX-week read is: expect positioning to tighten and pin into Friday, watch the biggest strikes as magnets, and be ready for the character to change the following week as gamma rolls off. It's one more piece of context in the full dealer-positioning read — the calendar layer sitting under the daily map.