The purpose of this study is to assess the long-term safety and effectiveness of odevixibat in participants with Alagille syndrome (ALGS). The participants of this study will have ALGS a rare genetic disorder that can affect multiple organ systems of the body including the liver, heart, skeleton, eyes and kidneys. Common symptoms, which often develop during the first three months of life, include blockage of the flow of bile from the liver (cholestasis), yellowing of the skin and mucous membranes (jaundice), poor weight gain and growth and severe itching (pruritis). Detailed Description: This Phase 3, open-label, multi-center extension study will have two groups of participants: Cohort 1 (participants who participated in Study A4250-012 [NCT04674761; ASSERT] and meet the entry criteria for this study) and Cohort 2 (infants under 12 months of age) with ALGS. The study will consist of 2 or 3 periods: A ‘Treatment period’ of 72 weeks (cohort 1) or 12 weeks (cohort 2). Participants will visit the clinic every 4 to 12 weeks and will receive a dose of 120 ?g/kg odevixibat daily. An ‘Optional extension period’ where participants who wish to continue receiving odevixibat after the ‘treatment period’ will have the opportunity to remain on treatment with visits every 16 weeks until the drug is commercially available. The optional extension is available provided continued use is supported by the risk-benefit profile, the participant has not been previously withdrawn or discontinued from the study, and the study is not terminated by the Sponsor. A ‘Safety follow-up period’ of 4 weeks (cohort 1) or 2 weeks (cohort 2). The Safety Follow-up Period will not occur for those who remain on treatment in the optional extension period. Participants will need to complete an e-diary and questionnaires throughout the study (cohort 1 only). Participants will undergo blood samplings, urine collections (cohort 1 only), physical examinations, and clinical evaluations. They may continue some other medications, but the details need to be recorded.