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Background: Small airways are considered the major site of airflow limitation in COPD patients. The forced oscillation technique has been successfully used as a measure of the airway resistance heterogeneity and gas trapping. Methods: 40 patients with moderate-to-severe COPD were monitored with forced oscillation technique and spirometry. Patients were randomized in 2 groups: 20 to receive fixed dose once daily Indacaterol/Glycopyrronium (GroupA) and 20 received single Glycopyrronium (Group B). Lung function was measured at baseline and after 6 months. The oscillometry parameters were the measure of resistance in the airways at 5 Hz (R5), at 20 Hz (R20), and lung reactance (X). Results: After the bronchodilator therapy, FEV1 and Inspiratory Capacity increased in both groups (Group A ΔFEV1 = 7,67%; Group B ΔFEV1 = 5,7%; Group A ΔIC = 15,72%; Group B ΔIC = 4,88%). Furthermore, impulse oscillometry detected an improvement in lung function. In Group A, all impulse oscillometry parameters significantly improved at follow-up, with a decrease of 47,8% (p=0,017) in R5, and in R20 =23,7% (p=0,024), in X = 295,23% (p=0,05). In Group B, impulse oscillometry showed a milder decrease in R5 (6,55%), in R20 (13,3%) and in X (267,77%). Conclusions: The new bronchodilators significantly reduce bronchial obstruction in small airways too. The oscillometry demonstrated greater sensitivity compared with spirometry for monitoring outcome measures of airway obstruction and the effect of long term therapy. This technique should facilitate the early optimization of therapy and a more personalized medicine for COPD patients.