A Phase 2/3 Study of Osivelotor in Adult and Adolescent Participants With SCD

Inclusion Criteria: Part A and Part B: * Male or female with SCD, HbSS and HbSB-zero * Participants with Hemoglobin ≥ 5.5 and ≤ 10.5 g/dL during Screening and considered stable by the Investigator. * For participants taking hydroxyurea and/or L-glutamine, the dose must be stable for at least 90 days prior to signing the ICF or assent and with no anticipated need for dose adjustments during the study in the opinion of the Investigator. Part B: * Participants with SCD ages 12 years and older, inclusive at screening. * Participants with more than or equal to 2 and ≤ 10 VOCs within 12 months of Screening. OLE: \- Participants who have completed the Part B successfully will be eligible. Exclusion Criteria: Part A and Part B: * Participants who had more than 10 VOC within 12 months of screening * Female participant who is breastfeeding or pregnant * Participants who receive RBC transfusion therapy regularly or received an RBC transfusion ---for any reason within 90 days of Day 1 * Participants hospitalized for sickle cell crisis or other vaso-occlusive event within 14 days of signing the ICF or anytime during the screening period. * Participants in Sub-Saharan Africa or who have relocated from Sub-Saharan Africa within 6 months. OLE: * Have any unresolved clinically significant adverse event, laboratory abnormality, or safety finding from Part B that, in the opinion of the Investigator, increases the risk of study drug administration. * Met permanent treatment discontinuation criteria during Part B. * Have withdrawn consent or are unable to comply with study procedure