Obstructive sleep apnea (OSA) and obstructive lung diseases (OLD) are common and interdependent respiratory disorders, where one condition may contribute to the development and worsening of the other (OLDOSA syndrome). The term OLDOSA syndrome includes two different conditions: Overlap syndrome (OVS: OSA + chronic obstructive pulmonary disease, COPD) and Alternative Overlap syndrome (aOVS: OSA + Asthma). Data on the interactions between lung function and respiratory monitoring during sleep in OLDOSA patients are few and controversial. Our study aims to evaluate the impact of lung function impairment on sleep breathing disorders, paying attention to the lack of literature about comparisons between OVS, aOVS, and the impact of small airways disease (SAD) in these patients. In total, 101 patients with a diagnosis of OSA and asthma or COPD underwent pulmonary function tests (PFTs) and nocturnal home sleep cardiorespiratory monitoring (HSCM). Exclusion criteria: Obesity hypoventilation syndrome (OHS) and other non-respiratory sleep disorders. Sleep time with oxygen saturation below 90% (T90) was negatively correlated with forced expiratory volume in the first second, % of predicted (%FEV), forced vital capacity, % of predicted (%FVC), forced expiratory flow at 25-75% of the pulmonary volume, % of predicted (%FEF), and, after multivariable linear regression analysis, %FEF remained an independent factor for T90 with a negative correlation in mild and moderate OSA. Obstructive apnea index (oAI) and FEV/FVC were negatively correlated in mild and moderate OSA. OVS presented with more severe OSA (higher AHI, oAI, and T90) and SAD (lower FEF) compared to aOVS. This study highlights a possible interdependence between OLD and OSA; obstruction of the large and small airways at PFTs contributes to the worsening of these patients' nocturnal hypoxemia and obstructive events of the upper airway during sleep. Furthermore, this study shows that patients with OVS should be carefully monitored, as they present worse data at HSCM and have greater small airways involvement compared to aOVS.