Tongue deformation during mandibular advancement, as characterised by tagged-MRI, may be helpful to predict mandibular advancement splint (MAS) therapy for obstructive sleep apnea (OSA). 68 OSA patients untreated underwent a MRI scan prior to trialling a MAS and treatment outcome was determined. Preliminary results identified 3 possible tongue deformations (en bloc, oropharyngeal and minimal/backward) with variable impact on the upper airway size. Over all participants, tongue deformation was not associated with treatment outcome, but among obese participants, “en bloc” tongue deformation was associated with positive treatment outcomes. This may also improve the understanding of the mechanisms underpinning MAS therapy effectiveness.