OPEX & monthly expiration
Why the 3rd Friday matters, and how pinning works.
OPEX is the standard monthly options expiration, the 3rd Friday of each month. It is where the largest open interest concentrates, so it tends to have outsized influence on price. Weekly and daily expirations exist too, but the monthly cycle carries the most contracts.
Pinning
As monthly expiry approaches, price often gravitates toward — and pins at — strikes with huge open interest. This is a side effect of dealers hedging their gamma (explained in the GEX track). Sharp moves are also common once that pinning pressure releases after the close.
How the Arena marks OPEX
In the Arena options chain, any expiry that is the 3rd Friday of its month is tagged with a purple O so you can factor pinning into your trade.