Inclusion Criteria:  Is at least 18 years old at screening. Body weight is greater than 45 kg at screening. Has adequate acoustic windows to enable accurate TTEs Diagnosed with oHCM Has documented LVEF ≥ 55% at rest. Has a valid measurement of Valsalva LVOT peak gradient at screening Female participants must not be pregnant or lactating Able to understand and comply with the study procedures, understand the risks involved in the study, and provide written informed consent according to national, local, and institutional guidelines before the first study specific procedure.  Exclusion Criteria:  Participated in a clinical trial in which the participant received any investigational drug (or is currently using an investigational device) within 30 days prior to screening, or at least 5 times the respective elimination half-life (if known), whichever is longer. Causing cardiac hypertrophy in other reasons Previously participated in a clinical study with mavacamten. Hypersensitivity to any of the components of the mavacamten formulation. Current treatment (within 14 days prior to screening) or planned treatment during the double-blinded treatment with a combination of beta-blockers and verapamil or a combination of beta-blockers and diltiazem. Has been successfully treated with invasive septal reduction Has documented obstructive coronary artery disease Has known moderate or severe (as per investigator's judgment) aortic valve stenosis, constrictive pericarditis, or clinically significant congenital heart disease at screening. Has any acute or serious comorbid condition that, in the judgment of the investigator, could lead to premature termination of study participation or interfere with the measurement or interpretation of the efficacy and safety assessments in the study. History of malignant disease within 10 years of screening Has safety laboratory parameters outside normal limits at screening as assessed by the local laboratory Has a positive serologic test at screening for infection with human immunodeficiency virus, hepatitis C virus, or hepatitis B virus, with the exception of hepatitis B s-antibody positive, which is a marker of immunity. Known infection with COVID-19 (coronavirus disease 2019) within 90 days of screening. Has a history or evidence of any other clinically significant disorder, condition, or disease that, in the opinion of the investigator, would pose a risk to participant safety or interfere with the study evaluation, procedures, or completion. Prior treatment with cardio toxic agents Unable to comply with the study requirements, including the number of required visits to the clinical site Is a first degree relative of personnel directly affiliated with the study at the clinical study site, any study vendor, or the study sponsor. Identified as alcohol addicts.