There are run plays (a draw for example) where the o-line blocks like it's a pass. They can also block like it's a run for a play action pass and still give the qb enough time to throw the ball.

One thing that isn't clear on TV is just how hard it can be during a play to see just where the ball is. Linemen are huge and RBs are typically shorter guys. When the play goes off it's often hard to see exactly what is happening and the defense is reacting to cues more so than seeing right where the ball is at that time.