Objectives: Identify whether the application of blood flow restriction (BFR) during walking influences fraction of expired oxygen (FeO2) and carbon dioxide (FeCO2) measures, key variables in the calculation of oxygen consumption (V̇O2) via indirect calorimetry. Design: Randomised cross-over. Methods: On separate visits, sixteen participants completed four experimental sessions (order randomised), each comprising 10 min of treadmill exercise; i.e., with or without BFR (60% arterial occlusion pressure) combined with two different intensity levels (100% or 120% comfortable walking speed). For data analysis, walking speeds within the same condition (with or without BFR) were pooled, and the speed variance was controlled within the statistical model. The FeO2, FeCO2, V̇O2, volume of carbon dioxide production, minute ventilation (V̇E) and respiratory exchange ratio were extracted from the metabolic cart from the fifth min of the exercise period to the 3 min post-exercise. Measures were averaged across 2 min increments during exercise and 1 min increments post-exercise. Results: Condition × time interactions were observed for FeO2 (p < 0.01) and FeCO2 (p < 0.01). Post hoc analysis identified within the BFR condition an increase in FeO2 (p < 0.01) during the exercise period and for 2 min post-exercise, while FeCO2 was decreased (p < 0.01) during the exercise period and 1 min post-exercise. A main effect of BFR and time was observed for V̇E (p ≤ 0.044) and V̇O2 (p ≤ 0.01). Conclusions: The increase of FeO2 and decrease of FeCO2 during BFR walking likely reduces the validity of V̇O2 values calculated via indirect calorimetry.