Obstructive sleep apnea (OSA) is a growing medical problem. Given the severity of the condition and the associated health risks, it is important to treat the disorder early and effectively. Adenotonsillectomy (AT) is the standard therapeutic approach, but seems inadequate in 20-40% of children. In children with Down syndrome this number even increases up to 55%. Residual OSA post-AT is often due to persistence of functional disorders that are not cured after surgical intervention. Therefore, oromyofunctional therapy emerges as promising new treatment for pediatric OSA. Despite the growing interest in oromyofunctional disorders and oromyofunctional therapy in children with OSA with and without Down syndrome, high-quality evidence is still lacking. In the first part of the project a case control study will be conducted to determine oromyofunctional disorders in children with OSA. The second aim is to investigate the short- and longer-term effects of oromyofunctional therapy on OSA severity, symptoms and quality of life using a randomized sham-controlled trial. In the third part of this project, the effectiveness of oromyofunctional therapy will be evaluated in children with Down syndrome and OSA using a one group pretest posttest design. This innovative research will lead to important new insights in oromyofunctional disorders and therapy in children with OSA and will be the start of a new evidence-based treatment method for pediatric OSA.