OPEX
Monthly options expiration, the third Friday of the month, when the standard monthly equity and index options expire.
OPEX is trader shorthand for the monthly options expiration. Standard monthly equity and index options expire on the third Friday of the month, or on the business day before it when the exchange is closed that Friday. Weekly and daily expirations now exist for many underlyings, but the third Friday still concentrates the standard monthly contracts, so open interest in those series ends on the same day.
Research has documented that closing prices of optionable stocks tend to cluster at strike prices on expiration days, which is called pinning. The hedging that option market makers held against the expiring contracts also ends with them.
Illustrative example (hypothetical numbers, not a real trade): in a month whose first day is a Wednesday, the Fridays fall on the 3rd, 10th and 17th, so OPEX is the 17th. A trader holding a monthly call that expires that day has until the close on the 17th to sell or exercise it.