Opioid use disorder is a major cause of drug-related harm and mortality. These can be reducedby expanded access to evidence-based and highly effective opioid agonist maintenance treatmentor therapy (OMT). There is a lack of consensus on how to assess opioid use disorder treatmentoutcomes, and key health outcomes are often omitted. We report the results of a Delphi studyto produce service user- and public health–centred international consensus guidance for OMToutcomes monitoring. An international group of 110 substance use specialists in 32 countries,including service providers, researchers and people with lived experience of OMT, produceddraft guidance over multiple meetings. The guidance includes a service user-reported OMT out-comes questionnaire, based on 26 core questions, plus optional questions, in six domains (treat-ment, physical health, mental health, social functioning, substance use, quality of life). A Delphipanel of 757 OMT professionals and service users (46%) from 29 countries, of which 40% werefemale, reviewed the questionnaire over two survey rounds, supporting and improving it (round 2mean agreement score on a 1-6 Likert scale: 5.2; 95%CI 5.1–5.3). By focusing on service user–reported and public health–centred outcomes of OMT, the OPTIMUS consensus guidance aims to facilitate the communication between service providers and service users and improve the qual-ity of care and the survival, health and quality of life of OMT service users.