||In this paper, we present a method to estimate oxygen uptake (V O2) during daily life activities and transitions between them. First, we automatically locate transitions between activities and periods of non-steady-state V O2. Subsequently, we propose and compare activity-specific linear functions to model steady-state activities and transition-specific non-linear functions to model non-steady-state activities and transitions. We evaluate our approach in study data from 22 participants that wore a combined accelerometer and heart rate (HR) sensor while performing a wide range of activities (clustered into lying, sedentary, dynamic/household, walking, biking and running), including many transitions between intensities, thus resulting in non-steadystate V O2. Indirect calorimetry was used in parallel to obtain V O2 reference. V O2 estimation error during transitions between sedentary, household and walking activities could be reduced by 16% on average using the proposed approach, compared to state of the art methods.