The main purpose of this study is to evaluate the pharmacokinetics (PK) of ocrelizumab administered subcutaneously (SC) in children and adolescents aged 10 to <18 years with RRMS. The study consists of a 48-week treatment period, an Optional Ocrelizumab Extension (OOE) period of at least 48 weeks, and Safety Follow-up (SFU) for 104 weeks.

For participants who are under 18 years old at the end of the OOE period, it may be extended until the participant turns 18 years old (or as required per local regulation) or until commercial ocrelizumab intravenous (IV) is approved for children and adolescents and available in the country for these participants, whichever occurs first.

Inclusion Criteria:

• Children and adolescents from 10 years to less than 18 years of age, at the time of baseline visit
• Body weight ≥25 kg
• Diagnosis of RRMS in accordance with the International Pediatric Multiple Sclerosis Study Group (IPMSSG) criteria for pediatric MS, Version 2012, or McDonald criteria 2017 or 2024
• Neurologic stability for at least 30 days prior to screening, and between screening and baseline
• Expanded Disability Status Scale (EDSS) score, 0-5.5, at screening
• Must have received all childhood required vaccinations as per local/national recommendations for childhood vaccination against infectious diseases

Exclusion Criteria:

• Participants who are positive for aquaporin 4 (AQP4) or myelin oligodendrocyte glycoprotein (MOG) antibody are not eligible to participate in the study
• Any known presence or suspicion of other neurologic disorders that may mimic multiple sclerosis (MS)
• History or known presence of recurrent or chronic infection (e.g., human immunodeficiency virus [HIV], syphilis, tuberculosis [TB])
• Contraindications against SC injections or other conditions not suitable for SC injections, e.g., extremely thin SC fat layer
• History of a severe allergic or anaphylactic reaction to humanized or murine monoclonal antibody or known hypersensitivity to any component of ocrelizumab solution
• Contraindications to mandatory premedications (i.e., corticosteroids and histamines), including closed-angle glaucoma for antihistamines
• Participants who have previously received treatment with B cell-targeted therapies, including ocrelizumab
• Any previous treatment with alemtuzumab, anti-CD4, cladribine, mitoxantrone, daclizumab, laquinimod, total body irradiation, or bone marrow transplantation
• Treatment with any investigational agent within 24 weeks of screening or 5 half-lives, whichever is longer (or longer if indicated by the PD action of the drug)

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