Just watched that PI call several times. I think it's a bad call. Jackson extends his arm, yes, but I don't see him pushing off. There was barely any contact between his hand and the defender. The defender's arms first get tangled with Jackson's and then Jackson is trying to free himself of the defender.
I've seen plenty worse, even in front of the official, that isn't called OPI. You have to let them play that out, the defender made contact earlier in the route too. Also, the ref didn't reach for the flag until after Jackson caught the TD.
But, at the most, it cost Seattle 4 points. The one that stands out is the holding call and subsequent blocking call on Hasselbeck which potentially resulted in a 14 point swing.