The goal of this randomized crossover trial is to investigate the effect of elastic bands attached to oral appliances to minimize mouth opening during sleep in obstructive sleep apnea (OSA) patients. The main questions it aims to answer are:

• Are the success rates (>50% reduction of AHI) of oral appliances with elastic bands superior to oral appliances treatment without elastic bands in treatment of moderate and severe OSA?
• Are there predictors that can identify patients that will benefit from use of elastic bands in oral appliance treatment of OSA?
• Are there predictors that can identify patients that are classified as non-responders to oral appliance treatment in general, both with and without elastic bands?

Participants will be treated with oral appliances with and without elastic bands for 3 weeks, in randomized order. At the end of each 3-week period, the effect of the treatment will be investigated with sleep registrations and questionnaires.

After the completion of both 3-week periodes, patients will continue using their preferred treatment modality (with or without elastic bands) and the oral appliance will be titrated if suboptimal treatment effect.

69 patients with moderate or severe OSA referred to Center of Sleep Medicine, Haukeland University Hospital (HUS), for oral appliance treatment will be invited to participate in the study.

The oral appliances will be produced with 63% and 69% of maximal protrusion for patients with moderate and severe OSA, respectively. Patients will be randomized to treatment with or without elastic bands before the oral appliance is fitted to the patient.

3 weeks after delivery, the subjective and objective effect of the treatment are investigated with questionnaires and home sleep testing with polygraphic sleep apnea recordings. The sleep recordings will be manually scored using scoring rules in accordance with the 2012 AASM guidelines. All sleep recording will be scored by the sane technician blinded to the treatment modality (with or without elastics).

Following, the patients will change treatment modality (with or without elastics), with identical investigation of objective and subjective treatment effect after 3 weeks of treatment.

If patients are not able to use their oral appliance due to side effects/discomfort at any point during the 3 week period, the issues are handled adequately, and control of the treatment is postponed for 2 week if remaining time to the planned control is <2 weeks. Patients are encouraged to make contact if side effects/discomfort related to the treatment are experienced.

After both treatment modalities (with and without elastic bands) are tested for 3 weeks, patients choose what treatment modality they want to continue treatment using, based on adherence, objective and subjective treatment effect. Treating physician will advice patients to choose the treatment that reduced their apnea-hypopnea index the most.

Objective treatment outcomes are classified following established success criteria:

• Success criterion 1: AHI < 5
• Success criterion 2: AHI < 10 and > 50% reduction of baseline AHI
• Success criterion 3: ≥50% reduction of baseline AHI
• Failure: <50% reduction of AHI from baseline

If success criterion 1 or 2 is not reached using the preferred treatment modality during the initial 3 week period, the oral appliance is titrated until it considered optimal. Following, the objective and subjective effect of the treatment is investigated with questionnaires and sleep recordings similar to initial treatment.

The patients with persistent <50% reduction of AHI from baseline after the oral appliance is optimally titrated will be offered treatment with CPAP.

Inclusion Criteria:

• Moderate or severe OSA
• Non-compliant and/or non-responder to CPAP-treatment
• Considered for treatment with Somnodent Fusion (second line oral appliance - indicated when retention on remaining teeth is insufficient to retain Narval CC)
• Participate in the "Sleep registry" at Center for Sleep Medicine, Haukeland University Hospital

Exclusion Criteria:

• Mild OSA or no OSA
• Considered for treatment with Narval CC (first line oral appliance)
• Patients with insufficient amount of teeth to retain an oral appliance
• Patients with full denture
• Patients that do not speak and/or read Norwegian language
• Patients that are not competent to give informed consent

The technician scoring the sleep recordings investigating the objective effect of oral appliance treatment with or without elastic bands, will be blinded to which treatment modality the patients are using.