A Study to Evaluate the Efficacy and Safety of Fenebrutinib Compared With Ocrelizumab in Adult Participants With Primary Progressive Multiple Sclerosis

Inclusion Criteria: * For sites in Germany and Italy only, enrollment is restricted to participants aged 46-65 years * A diagnosis of PPMS in accordance to the revised 2017 McDonald Criteria (Thompson et al. 2018). * Disability progression in the 12 months prior to screening. * Expanded Disability Status Scale (EDSS) score from 3.0 to 6.5 inclusive at screening. * Pyramidal functional subscore \>=2 at screening. * For participants currently receiving proton pump inhibitors (PPIs), H2-receptor antagonists (H2RAs), symptomatic treatment for MS (e.g. fampridine, cannabis) and/or physiotherapy: treatment at a stable dose during the screening period prior to the initiation of study treatment and plans to remain at a stable dose for the duration of study treatment. * Neurologically stable for at least 30 days prior to randomization and baseline assessments. * Ability to complete the 9-Hole Peg Test (9-HPT) for each hand in \<240 seconds. * Ability to perform Timed 25-Foot Walk Test (T25FWT) in \<150 seconds. * For female participants of childbearing potential: agreement to remain abstinent (refrain from heterosexual intercourse) or use contraceptive measures, and refrain from donating eggs. * For male participants: agreement to remain abstinent (refrain from heterosexual intercourse) or use contraceptive measures, and refrain from donating sperm. OLE Inclusion Criteria: * Completed the Double-Blind Treatment (DBT) phase of the study (remaining on study treatment; no other Diseas…