Introduction: Chronic Obstructive Pulmonary Disease [COPD] is associated with predominantly smaller airways and lung parenchyma involvement, whereas, Asthma involves predominantly smooth muscles located in the larger airway. Spirometry involves flow volume studies which are effort and coordination dependent and Impulse Oscillometry [IOS] is done during spontaneous respiration and hence effort independent. We studied the site of airway obstruction in COPD and Asthma patients using both Impulse Oscillometry and flow volume loop and whether both methods of pulmonary function testing are comparable with each other. Aims: To study site of airway obstruction using Impulse Oscillometry and Flow volume loop in COPD and Asthma patients. Methodology: The study was performed in 50 patients who were either COPD or Asthma. The patients underwent spirometry and impulse oscillometry. Both the modalities were analyzed for detecting the site of obstruction as per the diagnosis. Results and Conclusion: As per IOS, all the patients having large airway obstruction were asthmatics, all patients with mixed airway disease were COPD and 97.1% patients having small airway obstruction were COPD. Spirometry could not detect site of obstruction in 5 (10%) patients, on the other hand IOS could detect site of obstruction in all the 50 (100%) patients. Hence, Oscillometry is a better tool to know site of obstruction as compared to Spirometry in patients of Asthma and COPD.