Neuromyelitis optica spectrum disorder (NMOSD) is a severe inflammatory disease of the central nervous system characterized, in particular, by disabling episodes of optic neuritis and longitudinal extensive transverse myelitis. Its main pathogenic characteristic is the presence of anti-aquaporin-4 antibodies (AQP4-Abs) in the serum of affected patients. However, a proportion of patients with the typical NMOSD phenotype are, in fact, negative (seronegative) for AQP4-Abs and, within this category of patients, a proportion of them instead express antibodies to myelin oligodendrocyte glycoprotein (MOG-Abs). The presence of MOG-Abs in the sera of seronegative NMOSD patients is more frequently associated with monophasic disease and moderate symptom severity, and also appears to predict a better outcome. The present report is a review of the clinical and immunological features of MOG-Ab-positive NMOSD.