Background Continuous positive airway pressure treatment has been shown to lower blood pressure (BP) in patients with obstructive sleep apnea (OSA). The aims of the present pilot study were to evaluate the potential effects of oral appliance (OA) therapy on BP, to assess various outcome BP measures, and to inform sample size calculation. Methods Seventy-two patients with OSA and hypertension were randomly assigned to intervention with either an OA with mandibular advancement (active group) or an OA without advancement (control group). Before and after 3 months of treatment, the patients underwent nocturnal somnographic registration and 24-h ambulatory BP monitoring. Results Among the various BP measures, the largest trend toward effect of OA treatment was seen in 24-h mean systolic BP with a 1.8 mmHg stronger BP reduction in the active group compared with controls. A stronger trend toward effect was seen in a subgroup with baseline ambulatory daytime mean systolic BP >135/85 mmHg where the mean systolic BP fell, on average, 2.6 mmHg. Additional exclusion of patients with baseline apnea hypopnea index (AHI) ≤15 gave a significant reduction in mean systolic BP of 4.4 mmHg (P = 0.044) in the active group compared with controls. Conclusions In patients with OSA and hypertension, OA treatment had a modest trend toward effect on reducing BP. A stronger trend toward treatment effect was seen after excluding patients with normal baseline ambulatory BP. Additional exclusion of patients with baseline AHI ≤15 showed a significant treatment effect. Data to inform sample size for an adequately powered randomized study are provided.