Background <p>Lung oscillometry is an emerging investigation for assessing patients with obstructive airway disease. Comparison of oscillometry parameters with bronchodilator responsiveness (BDR) between patients with bronchial asthma and chronic obstructive pulmonary disease (COPD) with comparable severity of airflow obstruction has not been previously investigated. This study aimed to compare oscillometry parameters and bronchodilator responsiveness with other lung function parameters between stable bronchial asthma patients with airflow obstruction and COPD patients with a similar severity of airflow obstruction.</p> Methods <p>We retrospectively reviewed the data of 467 consecutive adults with clinical diagnoses of bronchial asthma (<i>n</i> = 187) and COPD (<i>n</i> = 280). Oscillometry, spirometry, and body plethysmography were performed before and after inhalation of 400&#xa0;µg of salbutamol. The patients were stratified based on the severity of airflow obstruction on spirometry. The z-scores of oscillometric parameters were used for comparison.</p> Results <p>The average age of the study population was 54.9 years, and 76.4% of the participants were male. Patients with COPD were older, had a higher proportion of smokers, and poorer lung function. The oscillometry parameters showed progressive impairment with increasing severity of airflow obstruction, irrespective of the underlying disease. Patients with bronchial asthma, particularly those with moderate to severe airway obstruction, demonstrated significantly greater R5 and R19 than patients with COPD. The within- and whole-breath X5 of bronchial asthma patients were not different from those of COPD patients with similar severity of airflow obstruction. Expiratory flow limitation at tidal breaths (ΔX5 &gt; 0.28&#xa0;kPa/L/s) was observed in both bronchial asthma and COPD patients across all severities of airflow obstruction. The proportion of BDR on oscillometry was significantly lower than that on spirometry for both bronchial asthma (35.3% vs. 57.1%; <i>p</i> &lt; 0.01) and COPD patients (19.3% vs. 47.1%; <i>p</i> = 0.02). BDR in spirometry and BDR in oscillometry showed fair agreement for bronchial asthma and a slight agreement for COPD patients.</p> Conclusions <p>Except for respiratory resistance, oscillometry parameters did not differ significantly between patients with bronchial asthma and those with COPD of similar severity of airflow obstruction. COPD patients exhibited a lower BDR in both spirometry and oscillometry than those with bronchial asthma.</p>