• Age 18 to 74 years, inclusive, at the time of informed consent

• Have a diagnosis of AQP4 antibody seropositive NMOSD according to the International Panel for NMO Diagnosis (IPND) criteria

• For women of childbearing potential: agreement to either remain abstinent (refrain from heterosexual intercourse) or to use reliable means of contraception (physical barrier \[patient or partner\] in conjunction with a spermicidal product, contraceptive pill, patch, injectables, intrauterine device or intrauterine system) during the treatment period and for at least 3 months after the last dose of study drug Cohort 1 (treatment-naïve NMOSD patients)

• Confirmation of NMOSD diagnosis with AQP4+ antibodies

• Have clinical evidence of at least 1 documented attack or relapse (including first attack) in the last year prior to screening

• Naive to maintenance therapy (disease-modifying therapy \[DMT\] or immunosuppressive therapy \[IST\]) Cohort 2 (NMOSD patients with inadequate response to RTX \[or its biosimilar\])

• Confirmation of NMOSD diagnosis and AQP4+ antibodies in the disease history of the patient

• Have a length of disease duration from first symptom of ≤5 years

• History of ongoing treatment with RTX (or its biosimilar) (at least 2 infusions) for NMOSD with a maximum duration of 6 months since last administration prior to enrolment in the study

• Ongoing disease activity after last RTX (or its biosimilar) infusion i.e., relapse and/or any new inflammatory event, confirmed by magnetic resonance imaging (MRI) or ophthalmological assessment