Patients in Opioid Substitution Treatment (OST) have increased mortality and morbidity, with circulatory conditions suggested to be a contributing factor. Since OST patients tend to have unmet physical healthcare needs, a small-scale intervention providing on-site primary healthcare (PHC) in OST clinics was implemented in Malmö, Sweden in 2016. In this study, we assessed registered circulatory conditions and healthcare utilization in OST patients with and without use of on-site PHC. Patients from four OST clinics in Malmö, Sweden, were recruited to a survey study in 2017–2018. Medical records for the participants were retrieved for one year prior to study participation (n = 192), and examined for circulatory diagnoses, examinations and follow-ups. Patients with and without on-site PHC were compared through descriptive statistics and univariate analyses. Eighteen percent (n = 34) of the sample had 1≤ registered circulatory condition, and 6% (n = 12) attended any clinical physiology examination or follow-up, respectively. Among patients utilizing on-site PHC (n = 26), the numbers were 27% (n = 7) for circulatory diagnosis, 15% (n = 4) for examinations, and 12% (n = 3) for follow-up. OST patients seem underdiagnosed in regard to their circulatory health. On-site PHC might be a way to diagnose and treat circulatory conditions among OST patients, although further research is needed.