The o.i. of a specific strike will be more important on the 3-month expiry dates... March, June, Sept, Dec.... Probably even more for the ones that have been open for trading for longer... typically June and Dec. Big o.i. will tend to attract the stock or index to that level on expiry. And when it's hovering close to it, it even happens to expire exactly on the strike-level. Look at it this way, if you're long on a strike and we're trading near it... you gamma trade around the strike. So you buy delta's (stocks/futures) below it and sell above it. But when you're short... you try to trade less... otherwise you're constantly losing on negative scalps... So the longs drive the underlying towards the strike... I've even traded in the opposite way as I theoretically should have when I traded short gamma close to expiry... also driving the stock towards the strike... There's a lot going on at major expiry dates....