Background Antibodies against myelin oligodendrocyte glycoprotein (MOG-IgG) have been reported in patients with aquaporin-4 antibody (AQP4-IgG)-negative neuromyelitis optica spectrum disorders (NMOSD). The objective of this study was to describe optic neuritis (ON)-induced neuro-axonal damage in the retina of MOG-IgG-positive patients in comparison with AQP4-IgG-positive NMOSD patients. Methods Afferent visual system damage following ON was bilaterally assessed in 16 MOG-IgG-positive patients with a history of ON and compared with that in 16 AQP4-IgG-positive NMOSD patients. In addition, 16 healthy controls matched for age, sex, and disease duration were analyzed. Study data included ON history, retinal optical coherence tomography, visual acuity, and visual evoked potentials. Results Eight MOG-IgG-positive patients had a previous diagnosis of AQP4-IgG-negative NMOSD with ON and myelitis, and eight of (mainly recurrent) ON. Twenty-nine of the 32 eyes of the MOG-IgG-positive patients had been affected by at least one episode of ON. Peripapillary retinal nerve fiber layer thickness (pRNFL) and ganglion cell and inner plexiform layer volume (GCIP) were significantly reduced in ON eyes of MOG- IgG-positive patients (pRNFL = 59 ± 23 μm; GCIP = 1.50 ± 0.34 mm3) compared with healthy controls (pRNFL = 99 ± 6 μm, p < 0.001; GCIP = 1.97 ± 0.11 mm3, p < 0.001). Visual acuity was impaired in eyes after ON in MOG-IgG-positive patients (0.35 ± 0.88 logMAR). There were no significant differences in any structural or functional visual parameters between MOG-IgG-positive and AQP4 -IgG-positive patients (pRNFL: 59 ± 21 μm; GCIP: 1.41 ± 0.27 mm3; Visual acuity = 0.72 ± 1.09 logMAR). Importantly, MOG-IgG-positive patients had a significantly higher annual ON relapse rate than AQP4-IgG-positive patients (median 0.69 vs. 0.29 attacks/year, p = 0.004), meaning that on average a single ON episode caused less damage in MOG-IgG-positive than in AQP4-IgG- positive patients. pRNFL and GCIP loss correlated with the number of ON episodes in MOG-IgG-positive patients (p < 0.001), but not in AQP4-IgG- positive patients. Conclusions Retinal neuro-axonal damage and visual impairment after ON in MOG-IgG-positive patients are as severe as in AQP4-IgG- positive NMOSD patients. In MOG-IgG-positive patients, damage accrual may be driven by higher relapse rates, whereas AQP4-IgG-positive patients showed fewer but more severe episodes of ON. Given the marked damage in some of our MOG-IgG-positive patients, early diagnosis and timely initiation and close monitoring of immunosuppressive therapy are important.