IInclusion criteria  Aged ≥ 18 years (no upper age limit); Current diagnosis of DSM-5 OUD via SCID-5-RV (moderate-severe at baseline for current episode); Currently enrolled on Met (30mg/day or less) or sublingual Bup or Bup-NX (24mg/day or less) or Esp (18mg/day or less) and in the view of the clinician would be able to convert to XR-Bup within 7 days post randomisation; Voluntarily seeking treatment and able to attend the clinic as required in the protocol; Able to communicate in English to level required to accept standard care and psychosocial intervention; Possession of a contactable personal mobile phone or landline telephone number and ability to nominate at least one locator individual with a verifiable address and a telephone number to assist with the arrangement of follow-up appointments; Living circumstances judged to be of sufficient stability to be able to engage/adhere to the study protocol; Is not pregnant (confirmed) or breast feeding and, if currently or intending to have potentially procreative intercourse, agrees to use a birth control method (either oral hormonal contraceptives, barrier [condom or diaphragm], or Nexplanon implant) for the duration of the study.  8.2 Exclusion criteria  Clinically significant medical condition or observed abnormalities on physical examination or laboratory investigation, including but not limited to:  uncontrolled hypertension, significant heart disease (including angina and myocardial infarction in past 12 months), or any cardiovascular abnormality which is judged to be clinically significant; severe alcohol dependence/withdrawal syndrome which is judged to be clinically significant and may constitute a risk to the patient's safety; acute hepatitis taken as clinical jaundice on examination, or evidence of blood bilirubin level above the normal range for local reference criteria, or evidence of serum levels of aspartate aminotransferase, alanine aminotransferase levels that are more than three-times the upper limit of the normal range; History of allergic or adverse reactions to Bup or the proprietary ATRIGEL delivery system for XR-Bup (Sublocade®)*; Clinically significant or uncontrolled mental health problems (including but not limited to psychosis, bipolar disorder, schizoaffective disorder), or history or evidence of organic brain disease or dementia that may compromise safety or compliance with the study protocol; Current (past 30 day) suicide plan or suicide attempt in past six months; Current criminal justice involvement with legal proceedings, which in the opinion of a medically qualified investigator indicates a risk that the patient would fail to complete the study protocol due to re-incarceration or move away from the centre's catchment area. Currently taking oral or depot naltrexone therapy or enrolment in any form of naltrexone therapy within 90 days prior to study screening;  Any contraindication to Bup*.  Participant is ineligible if they have any allergic or adverse reactions or contraindication to Buprenorphine. If participant has any allergic or adverse reaction or contraindication to Met or naloxone, or excipients of Bup-NX or Esp they can be prescribed Bup within the trial.