The number of optical coherence tomography (OCT) examinations in substance use disorders is gradually increasing. However, OCT findings in opioid use disorder (OUD) have not yet been investigated. In this study, we compared the retinal nerve fiber layer (RNFL), the ganglion cell layer (GCL), the inner plexiform layer (IPL), and choroid thickness (CT) of OUD and control groups. We included 43 male patients and 43 healthy male controls of similar age (p = 0.296) in the study, prospectively. On the day of OCT application, urine toxic screening test results of all OUD patients were positive for opioid use. There was a significant difference between OUD and control groups in terms of CT (p < 0.05), nasal superior (NS), and nasal (N) sectors of the RNFL (p < 0.05) values of both eyes. According to the binary logistic regression analysis, the sensitivity of mean NS (p = 0.001) and mean CT (p = 0.007) related to the diagnosis of OUD was 72.1 percent, and the specificity was 65.1 percent. Receiver operating characteristic (ROC) analysis revealed that the sensitivity and specificity of mean CT for the diagnosis of OUD were 18.6% and 97.7%, respectively. This is the first study to investigate the OCT findings in OUD. Our findings are important in terms of showing thinning in the choroidal layer and an increase in the volume of the NS and N sectors of RNFL while detecting opioids in the body/urine. Further studies are needed to clarify whether these differences are due to the acute and/or chronic effects of opioids.