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Background Opsoclonus-myoclonus syndrome (OMS) is a rare neuroimmunologic disorder characterized by opsoclonus, myoclonic jerks mostly in the face and limbs, cerebellar ataxia, tremors, and encephalopathy. OMS is rare in adults and exceedingly rarer in pregnancy, as only a few cases in pregnancy have been reported. We present what we understand is the first case of postpartum OMS. Methods and results We report and discuss a challenging case of OMS which started 6 weeks postpartum. Despite extensive infectious and malignancy evaluation, an underlying etiology was not readily apparent thus we treated her with high dose intravenous steroids and intravenous immunoglobulin (IVIG) for presumed idiopathic autoimmune OMS. She relapsed and additional workup identified new enhancing lesion on MRI brain, positive MOG-IgG, and CSF negative for oligoclonal bands. She was transitioned to maintenance IVIG and ultimately to rituximab with better results. At 2 year follow up her exam was improved and without objective evidence of abnormal movement or opsoclonus on maintenance Rituximab infusion 1,000 mg every 6 months. Conclusion In OMS, an autoimmune response is usually thought to occur by molecular mimicry with neuronal cell surface antigens in association with infections. Since a preceding infection was absent in this case, we propose that the immune response here was initiated due to immunological changes in pregnancy and postpartum period possibly due to fetal tissue exposure (fetal microchimerism). The presence of the MOG antibody raises the possibility that OMS is another clinical manifestation of MOG-associated disease (MOG-AD), which in our case is supported by characteristic CSF and radiographic findings of MOG-AD.