Forced oscillation technique (FOT) is a method of measuring lung obstruction of central airways (resistance, Rrs) and elastic properties/distal airway ventilation (reactance, Xrs), which is different from spirometry since it is performed during tidal breathing, utilizing various sound frequencies (5, 11, and 19 Hz) to separate out regions of the lungs and isolates breathing cycles (inspiration and expiration). PURPOSE: To evaluate FOT metrics of lung mechanics and obstruction within a chronic obstructive pulmonary disease (COPD) population based upon severity (mild [MLD], moderate [MOD], and severe [SEV]). METHODS: Seventeen COPD, and fourteen healthy (H) patients (age: 69.5±5.8 vs. 49.3±17.6 yr.*; height: 169.9±11.3 vs.163.7±18.1cm; weight: 77.8±18.8 vs. 83.3±32.4 kg, COPD vs. H respectively, *p<0.05) completed spirometry and FOT measurements. COPD participants completed the St. George Respiratory Questionnaire (SGRQ) and severity was classified based on GOLD spirometry (MLD, MOD, and SEV) RESULTS: In those with MOD&SEV COPD, total Rrs and Xrs at all frequencies, except expiratory R11, were significantly different (p<0.05) from H, but only R19 was different for MLD participants. R5: 3.9±1.1 v 3.1±1.3, 5.9±2.5*cmH2O/L/s; X5: -1.1±0.9, -1.4±0.6, -4.8±3.2*cmH2O/L/s; R19: 0.3±0.07, 2.4±0.9*, 3.3±1.0*cmH2O/L/s; X19: 0.4±0.03, -0.2±0.7, -1.1±1.0*cmH2O/L/s, for H, MLD, and MOD&SEV groups respectively, *p<0.05 vs. H. No significant difference (p>0.05) between COPD severities were shown for Rrs or Xrs at 5, 11, or 19Hz. SGRQ scores were not different based on spirometric severity (M: 44.6±18.6; MOD: 42.9±15.2; S: 43.8±11.8). CONCLUSION: FOT metrics made distinctions between COPD MOD&SEV and H, as well as MLD and H for mixed respiratory tract obstruction. However, no distinctions between COPD severities could be made. Obstruction increased with severity highlighting that more heterogeneous total airway obstruction (frequency dependent) is observed with mild severity patients, followed by more homogeneous obstruction as severity increased. Elastic properties and efficiency of ventilation decreased (became more negative) with increased COPD severity as airflow to and from distal airways become more limited due to hyperinflation from loss of alveolar support and elastic recoil.