The main objective of this study is to evaluate the efficacy of satralizumab compared with placebo based on time from randomization to the first occurrence of an adjudicated MOGAD relapse in the double-blind (DB) treatment period. Participants who experience an adjudicated relapse or complete the DB period can enter open-label extension (OLE) period. After the primary clinical cutoff date (CCOD), additional adolescent participants may be enrolled directly into the OLE period.

Inclusion criteria

• Participants who are aged >=12 years at the time of signing Informed Consent Form
• Confirmed diagnosis of MOGAD with a history of >=1 MOGAD relapse in the 12 months prior to screening or >=2 attacks in the 24 months prior to screening
• Expanded Disability Status Scale (EDSS) score of 0-6.5 at screening
• High-contrast visual acuity (HCVA) better than 20/800 in each eye at screening
• Participants receiving either no or ongoing chronic immunosuppressant treatment (IST) for MOGAD at the time of screening
• For women of childbearing potential: participants who agree to remain abstinent or use adequate contraception during the treatment period and for at least 3 months after the final dose of satralizumab

Exclusion criteria

• Presence of aquaporin-4-antibodies immunoglobin G (AQP4-IgG) in the serum
• History of anti-N-methyl-d-aspartate receptor (NMDAR) encephalitis
• Any concomitant disease other than MOGAD that may require treatment with ISTs or oral corticosteroids (OCS) or intravenous (IV) corticosteroids at doses > 20 milligrams (mg) prednisone equivalent per day for >21 days during the study
• Participants who are pregnant or breastfeeding, or intending to become pregnant during the study or within 3 months after the final dose of satralizumab
• Participants with active or presence of recurrent bacterial, viral, fungal, mycobacterial infection, or other infection at baseline
• Participants with evidence of latent or active tuberculosis (excluding patients receiving chemoprophylaxis for latent tuberculosis infection)
• Participants with positive screening tests for hepatitis B and C
• Receipt of live or live attenuated vaccine within 6 weeks prior to baseline
• History of severe allergic reaction to a biologic agent

For eligible studies, qualified researchers may request access to individual patient level clinical data. See Roche's commitment to transparency of clinical study information here: https://go.roche.com/data\_sharing