Oscillometry (OS) is typically measured seated. Body position (BoP) influence on PFTs is well known but not on OS. We wished to determine BoP effect on OS and if PFTs or lung density (LD) would predict any changes. COPD and heathy subjects (HS) performed OS (tremoFlo C-100, 5-37Hz, Thorasys) sitting, supine and sitting again, and OS was determined; resistance at 5 Hz (R5), frequency dependence of resistance (R5-19), reactance at 5 Hz (X5), resonance frequency (Fres) and reactance area (AX). Absolute change (Δ) and %change (Δ%) sitting to supine OS was calculated. PFTs and CT scan LD (15th percentile density+1000HU, AirwayInspector.acil-bwh.org) were measured. Sitting vs. supine OS was compared with Holm's corrected Student's t-tests. Δ and Δ% were correlated with PFTs and LD by linear regression. R5Δ correlated with TLC (r=0.55, p=0.03, n=15), R5-19Δ with TLC (r=0.85, p<0.001, n=15) and LD (r=0.67, p=0.03, n=10), R5-19Δ% with RV/TLC (r=0.64, p=0.01, n=15) and AXΔ% with LD (r=0.62, p=0.05, n=10). OS changes with BoP in both COPD and HS. BoP changes in R5, R5-19, and AX correlate moderately to very strongly with TLC, RV/TLC or LD. These findings impact on the use of OS in supine patients and supine CT data to model airways for comparison with sitting OS, and suggest airway-parenchymal interdependence influences airway caliber. Independent confirmation is necessary before using R5-19Δ as a predictor of TLC or LD.