Although Osteogenesis Imperfecta (OI) affects the connective tissue causing brittle bones with consequent skeletal deformities, it is important to go beyond bones, as OI might also affect other important functions. We have implemented a multidisciplinary study to assess lung function, breathing pattern, sleep quality and nutritional status in 27 adults OI type III and IV patients (median age: 34.6 years; 19 women; 14 type III). According to nocturnal oxygen desaturation, two groups were identified: 13 patients with (OI_OSA, incidence: 48.2%) and 14 without (no_OSA) obstructive sleep apnea. The former was characterized by higher spinal and ribcage deformity, more restrictive lung function, paradoxical thoracic breathing in supine position, rapid and shallow breathing, higher body mass index, longer neck and waist circumferences, higher abdominal volume and a greater percentage of body fat mass, particularly localized in the trunk. The best predictor of OI_OSA was the negative value of the supine ribcage contribution to tidal volume, followed by the neck-to-body height ratio. The pathophysiology of OI ensued a dangerous vicious circle in which breathing, sleep and nutritional status are tightly linked. The vicious circle is fed by some intrinsic characteristics of OI (thoracic, cranial and mandibular deformities) and some bad daily habits of the patients (physical inactivity and low dietary quality). The former restricts respiratory function, and the latter makes Olers more prone to experience overweight or obesity. The main consequence is a high incidence of obstructive sleep apnea, which remains an underdiagnosed disorder in individuals with severe OI who are obese, with a neck-to-height ratio over 31.6%, and characterized by paradoxical breathing in the supine position.