cmathews,
I think that you have OPI when the eligible A player (who is restricted from the snap) interferes with B's right to play the ball. In your example, would I call OPI? Don't know, would have to see it. I wouldn't call it just because B had some established position, although this could be a factor if A truly had created an advantage by running into B. In my experience, most of these end up being non-foul contact where both players are going for the ball. In this case unless there is obvious advantage, I let the play call itself. Incomplete, no flag.
Just as B can't play the "man", neither can A just play the "man". I believe it's incumbent on both A and B to make a bona fide effort to make a play on the ball to avert a possible interference call. If they are both going for the ball, contact is usually incidental (unless of course, you have an obvious hold or intentional contact).
I still remember on a downfield pass, A and B both had there eyes on the ball, running side by side, they both went up, tangled feet, fell down without any chance of playing the ball. I had a big no call and got BOOED by both sides for missing an obvious DPI and OPI at the same time. It doesn't get any better than that!!
