This is a Phase 3, multicenter study of OMS721 in adults and adolescents with atypical hemolytic uremic syndrome (aHUS). The uncontrolled, open-label study will evaluate the effect of OMS721 in subjects with plasma therapy-resistant aHUS and plasma therapy-responsive aHUS. This study has four periods: Screening, Treatment Induction, Treatment Maintenance, and Follow-up. Approximate enrollment is 80 subjects. An interim analysis will be performed after 40 subjects have completed 26 weeks of treatment for potential registration.