Classic Optimality Theory treats phonological computation as a one-step parallel evaluation of output candidates. Such an approach does not handle well phonological opacity, in which the context of phonological processes is obscured on the surface. This article shows two examples of phonological opacity: Tiberian Hebrew Spirantization and Polish o-Raising. While these mappings are impossible to account for in classic OT, they are readily analyzed using the framework of Derivational OT.