Receivers are not allowed to block downfield prior to the end of the pass. If they engage a defender to change his course or obstruct his movement, then it's OPI. If they just exist in space and make the defender go around him, then I've got nothing -- the receiver has the right to his position on the field.
This is also very much a judgement call and a "I've got to see it" play, so if you have something particular in mind, I can't internet officiate it without video (and even then, it might be tough).