This paper provides evidence for the activity of the Obligatory Contour Principle (OCP) as a constraint on dynamic alternations in the synchronic grammar of Qatari Arabic. It shows that the OCP is subject to proximity and to a gradient similarity effect. In Qatari Arabic, there are two variable phonological alternations that interact with the OCP, affrication and lenition. The velar stops /g/ and /k/ affricate to [ʤ] and [ʧ], respectively, when adjacent to [i(:)]. However, affrication is blocked when the outcome includes a sequence of segments that are highly similar. Lenition applies variably to the phoneme /ʤ/, which surfaces as [ʤ] or [j]. Usually, the probability of lenition applying to its eligible candidates is around the level of chance. The process, however, applies categorically when a violation of the OCP would otherwise be incurred. The data are analyzed within the framework of Optimality Theory.