DPI and OPIDefensive vs offensive interference
Defensive pass interference usually means the defender illegally restricted the receiver's chance to catch the ball. Common examples include grabbing an arm before the ball arrives, playing through the receiver's back, or hooking the receiver so their body turns away from the pass.
Offensive pass interference usually means the receiver or another offensive player gained an unfair advantage. Common examples include a clear push-off to create separation, blocking downfield before the pass is thrown or arrives, or deliberately screening a defender on a pick route beyond the allowed contact area.
Common argument"The defender never looked back"
Not turning to look for the ball can make the defender's actions look worse, but it is not always a foul by itself. Officials still need meaningful restriction before calling pass interference. A defender who maintains legal position without grabbing, blocking, or playing through the receiver may avoid a flag even without turning their head.
The opposite is also true: looking back does not make illegal contact legal. If the defender hooks an arm, grabs the receiver, or drives through the body before the ball arrives, the fact that they were looking for the pass will not usually save the play.
EnforcementWhy penalties differ
Pass interference is one of the rule areas where league differences matter. In the NFL, defensive pass interference is commonly enforced at the spot of the foul with an automatic first down, while offensive pass interference is a yardage penalty from the previous spot. College, high school, Canadian, and flag football codes may use different yardage, automatic first-down, loss-of-down, or replay-review rules.
For a practical read, separate the foul from the penalty. The foul is about illegal restriction during a pass play. The penalty is whatever the governing rulebook for that game says to enforce.