Obstructive sleep apnea (OSA) may lead to increased circulating concentrations of inflammatory biomarkers and treatment may change these. We aimed to assess the effect of oral appliance (OA) therapy on inflammatory biomarkers in a randomised controlled pilot trial. A total of 71 patients with OSA and systemic hypertension were randomly allocated to an active, mandible protruded (OAa) or a passive, mandible non-protruded device (OAp) treatment. Serum concentrations of the inflammatory biomarkers white blood cells, high-sensitivity C-reactive protein, interleukin 6, interleukin 10, and tumour necrosis factor-alpha were measured at baseline and after 3 months of OA treatment. The differences between treatment groups in biomarker concentration change during the treatment were presented as the Vargha and Delaney effect size and evaluated with the Wilcoxon-Mann-Whitney test. This effect size expresses the probability of a higher value in a random participant from one group compared with a random patient from the other group, and a value of 0.5 means stochastically equal groups. After 3 months of treatment, there was a significant reduction of the apnea-hypopnea index in the OAa group compared with the OAp group (effect size 0.258, 95% confidence interval 0.146-0.386, p < .001). There were no significant differences between the groups in any of the inflammatory markers' concentration changes during the treatment period (effect sizes between 0.488 and 0.524; all p values >=.737). Thus, OA treatment for 3 months did not affect circulating concentrations of some common inflammatory markers in patients with OSA and systemic hypertension.