Nowadays, drug-induced sleep endoscopy is widely recognised as a valid tool for diagnosis and treatment planning of obstructive sleep apnoea syndrome (OSAS), as it allows a direct visualisation of sites and patterns of collapse of the upper airways (UA). Various classifications have been proposed in the literature to describe the events observed during DISE, including the NOHL (Nasopharynx cavity and walls, Oropharynx, Hypopharynx, Larynx) classification (Vicini 2012). This study was aimed at assessing which anatomical structures, according to the NOHL Classification, were most frequently involved in UA collapse in patients with moderate to severe OSAS, and evaluating treatment results with oral appliances (OA), in terms of AHI reduction. The study group consisted of 35 patients (29 M, 6 F, mean age 50.6, average BMI 26) with polysomnographic diagnosis of moderate to severe OSAS, subjected to DISE and classified according to the NOHL classification to identify the anatomical sites and patterns of UA collapse most frequently reported. Patients were subsequently addressed to mandibular advancing device (MAD) therapy and treatment results in terms of AHI reduction were evaluated. In the sample examined, the anatomical structures most frequently involved in the collapse of the UA were the nasopharynx cavity and walls and tongue base, with a correlation index of 0.35 (p < 0.04), while no significance was found for the retro-palatal area or larynx. Descriptive analysis revealed multilevel collapse in all patients, involving multiple anatomical structures in obstructive mechanics. In all patients, AHI reduction was observed after treatment with MAD (p < 0.00).