The point is not that the OUN was innocent but that Bandera did not inspire any of the massacres. He was, for one thing, Catholic and unlikely to be inspiring sectarian violence toward the Poles. Note that Poland today takes a very magnanimous attitude about the modern followers of Bandera's philosophy, apparently recognizing that Ukrainian nationalism is helpful to any nationalist, including a nationalistic Pole. It is especially salutary in order to balance the European community's leftist internationalism.
That land went back and forth between Poles and Ukraine/Russia for centuries.