What happens at monthly options expiration (OPEX)?

What actually expires, and when

Standard monthly equity options, including those on SPY and QQQ, stop trading at the close on the third Friday and settle on that day's closing price. If the third Friday is an exchange holiday, expiration moves to the preceding business day. In-the-money equity options are exercised automatically if they finish at least one cent in the money, unless the holder instructs otherwise, and the resulting share deliveries land in accounts over the weekend.

The standard monthly SPX options work differently. They stop trading at the close on Thursday and settle on Friday morning, to a special opening quotation calculated from the opening prices of every stock in the index. That means the settlement value can differ noticeably from both Thursday's close and any price you see on a chart Friday morning. The weekly SPX contracts that expire the same Friday, by contrast, settle on the Friday close.

Since daily expirations now exist on the major index products, the monthly date is no longer the only expiry that matters. It is still usually the largest single block of open interest in the month for single stocks, because many stocks list only monthly and weekly contracts and institutional positions are concentrated in the standard monthlies.

Why hedges rolling off matters

Every open option carries a hedge somewhere. When a dealer is long gamma at a heavy strike, that position has been dampening moves, selling rallies and buying dips, for days or weeks. When the option expires, the dampening disappears with it. When a dealer is short gamma, the amplification disappears.

So the question at OPEX is not whether the market rises or falls but whether the structure that was shaping its moves is about to change. If a large share of the positive gamma on the board belongs to the expiring monthly, the following week can feel looser, with wider ranges and fewer stalls. If most of the expiring open interest was negative gamma, the following week can feel calmer than the one before it.

The size of the shift depends on what remains. A month where the next expiries already carry heavy open interest near the price may see little change. A month where the expiring contracts held most of the book can see the map look very different on Monday.

The week into expiration

Two quieter forces are often discussed in the days before a monthly expiry. One is charm, the decay of out-of-the-money deltas with time, which under the usual assumption that dealers are short index puts produces dealer buying as those puts approach expiry. The other is vanna, the change in delta as implied volatility shifts, which pushes in the same direction if volatility drifts lower into the date.

Both depend on positioning assumptions and both are easily overwhelmed by news. It is more accurate to say expiration week has identifiable mechanical currents than to say it has a predictable direction. Market commentary that attributes every OPEX week rally to these flows is telling a story, not quoting a measurement.

Another effect is simpler. As expiration approaches, gamma at strikes near the price becomes very large, so hedging around those strikes becomes more active. That can make price action near heavy strikes look sticky late in the week, though a heavy strike sitting still is not, by itself, a reliable magnet.

Reading the book around OPEX

The useful habit is to look at the board twice: once with everything, and once with the expiring contracts removed. The difference tells you what the market's structure looks like the moment Friday ends. Levels that belong mostly to the expiring monthly are levels that will not exist next week.

It is also worth remembering that open interest reported on Friday morning is from Thursday's close. Positions closed or rolled on expiration day itself only show up in Monday's numbers, so the first clean view of the new structure comes after the weekend.