Local Optionality -- phonological variation that manifests differently at different loci within a single form -- poses a significant problem for versions of Optimality Theory (OT) with parallel evaluation, which predict that optionality should be global. In this paper, I propose a solution to that problem that combines a multiple-rankings theory of variation with Harmonic Serialism (HS), a derivational version of OT. In HS, Gen is restricted to performing one single change at a time, and a single form undergoes multiple passes through a Gen->Eval loop -- optimality is evaluated locally for each single change. When combined with a theory of variation in which constraint ranking may differ at each instantiation of Eval, this means that the ranking of variable constraints may differ at each step in the derivation. In a form with multiple loci, the choice of variant for each locus is therefore evaluated separately, giving rise to local optionality.