Introduction There is a variety of specialized outpatient and residential Alcohol and Other Drugs (AOD) services, but research on outcomes of these services is limited. Given the chronic, relapsing nature of AOD problems, there is a need for longitudinal research on outcomes after treatment in various AOD services. Patient-Reported Outcome Measures (PROMs) and Patient-Reported Experience Measures (PREMs) are hardly used in the AOD field but provide excellent tools and a framework to monitor progress and outcomes in these services based on experiences of service users. Objectives The objectives of the OMER-BE study are to: 1. Assess and compare patient characteristics at baseline in various treatment modalities 2. Test and prepare the routine measurement of PROMs and PREMs in AOD services using a self-report tool 3. Assess patient-reported experiences qualitatively in various treatment modalities for AOD patients The overall goal is to continuously assess and improve AOD services. Methods We have set up a naturalistic, longitudinal cohort study for which we will engage and follow up 250 AOD users as they present themselves in selected AOD services in four different treatment modalities (outpatient non-pharmacological treatment, outpatient substitution treatment, residential psychiatric treatment and therapeutic communities for addictions). Sociodemographic and clinical factors and PROMs will be assessed at baseline. PROMs and PREMs will be assessed at 45-, 90- and 180-days follow-up. The questionnaires that will be used during the baseline and follow-up assessments are based on the ICHOM Standard Set for Addictions (ICHOM SSA, 2020), a set of brief validated questionnaires to measure and monitor treatment outcomes routinely in AOD services. Following the 6-month follow-up we will perform a qualitative study in a subset of N=20 participants (5 per treatment modality). These participants will be invited to take part in an in-depth interview with one of the researchers, where the following topics will be discussed: ...