Introduction: The clinical benefit of bibloc over monobloc appliances has not been established in randomized trials treating obstructive sleep apnoea (OSA). We hypothesized that the two types of appliances are equally effective in treating moderate to severe OSA. Materials and methods: We performed a blinded, multicenter, randomized, controlled, prospective, parallel-group trial including patients aged 18 years or older who had moderate-to-severe OSA. Patients were randomly assigned to receive either a bibloc or a monobloc appliance with the intention to protrude the mandible 75% of the individual maximal protrusion capacity. At baseline a one-night respiratory polygraphy was done without any respiratory support. The polygraphy was iterated with the appliance in place at a 6-week follow-up. The primary outcome was the absolute change in the apnoea-hypopnea-index (AHI) from baseline to the 6-week follow-up, analysed in the per-protocol population. All patients who received an appliance were included in the safety analysis. This trial is registered with ClinicalTrials.gov, number NCT02148510, and approved by Uppsala Regional Ethical Review Board, Sweden (#2014/021). Results: We recruited patients from three dental specialist clinics in Sweden; enrolment of 302 patients was done between March 2014 and April 2016; 146 randomized to bibloc and 156 to monobloc appliance. Twenty-three patients in the bibloc group and 17 in the monobloc group were withdrawn due to reasons like appliance could not be fitted, lack of compliance, adverse events or non-valid follow-up polygraphy i.e. a per-protocol group of 123 bibloc and 139 monobloc treated patients. The mean change of AHI from baseline to 6 weeks of treatment was -13.8 (95% CI -16.1 to -11.5; p < 0.001) in the bibloc group and -12.5 (95% CI -14.8 to -10.3; p < 0.001) in the monobloc group. The mean difference was not significant between the groups (-1.3 (95% CI -4.5 to 1.9). The most common adverse event in the orofacial region was upper airway infection followed by complains from various parts of the mouth, jaws and teeth. Conclusions: Bibloc and monobloc appliance treatment was equal in their effects in treating OSA as measured by at home polygraphic respiratory measures and the appliances were associated with a similar degree of adverse events. Acknowledgements: Funding from Uppsala-Örebro Regional Research Council and Vastmanland County Council, Sweden.