OPEX (Options Expiration)

OPEX refers to scheduled options expirations - most notably the monthly third-Friday cycle and the quarterly 'triple witching' - when large blocks of open interest leave the board at once. Because expiring positions take their dealer hedges with them, the sessions around OPEX often reset the market's structure and can change its volatility regime.

Index options now expire daily, but the monthly and quarterly cycles still hold the largest and oldest open interest. When that OI expires, the gamma dealers were hedging around it disappears: pins release, walls dissolve, and whatever dampening or amplifying pressure the book was exerting resets.

This is why post-OPEX behavior is watched so closely. A market that was pinned under a heavy long-gamma book can move freely - in either direction - once the anchoring positions expire, and the days after a large expiration often show a change of character. The key question each cycle is whether positioning reloads at nearby strikes (regime continues) or fails to rebuild (structure thins).

Triple witching - the quarterly coincidence of index futures, index options, and stock options expiring together - is the largest scheduled turnover of open interest and hedging positions on the calendar.