Medicare and Opioid Safe Treatment Act or the MOST Act This bill establishes several requirements for the Centers for Medicare & Medicaid Services (CMS), and alters requirements under Medicare and Medicare Advantage, related to pain management and opioid use. For example, the bill requires certified opioid treatment program services to be covered under Medicare. Additionally, among other requirements, the CMS must review payments under Medicare for opioid and non-opioid pain management procedures, specifically with respect to ambulatory outpatient surgical procedures and hospital outpatient department services. The CMS must ensure that there are no payment incentives for using opioids instead of non-opioid alternatives and must make revisions accordingly. The bill also requires payment under Medicare to federally qualified health centers and rural health clinics that have health care practitioners who are newly certified to provide medication-assisted treatment (e.g., buprenorphine).