Distinguishing aquaporin-4-antibody(AQP4-Ab)-negative neuromyelitis optica spectrum disorders (NMOSD) from optico-spinal predominant MS is a clinical challenge with important treatment implications. 12 AQP4-Ab-negative patients presenting to an NMO clinic were selected for having overlapping features of NMO and MS. 27 NMO and MS experts reviewed their case vignettes, including relevant findings. Opinions were categorized for diagnosis (NMO, MS, indeterminate, other) and treatment (MS drugs, immunosuppression, none). The mean proportion of agreement for diagnosis was low (0.51). The majority opinion was divided between NMO versus MS (9 cases), monophasic longitudinally extensive transverse myelitis (LETM) versus NMOSD (1), acute disseminated encephalomyelitis versus NMO (1), and recurrent isolated optic neuritis (ON) versus NMOSD (1). LETM and/or ON with limited recovery even if accompanied by typical MS features (short TM, mild ON, oligoclonal bands), were associated with a low rate of MS diagnoses. TM with short lesions or mild ON did not preclude a diagnosis of NMO. Agreement on treatment was higher (0.64) with immunosuppression being the most common choice not only in patients with NMO but also in those indeterminate between NMO and MS. Experts frequently disagree on the diagnosis of NMO/MS overlapping syndromes. Prospective long-term studies on ultimate diagnosis in this group of patients are needed.