Using ambient pressure X-ray photoelectron spectroscopy, we were able to observe the process of oxidation of tantalum with different morphological parameters. Being able to trace surface evolution during oxidation, we evaluated the activation energy of oxidation under the influence of strain and grain boundaries. It was found that the metal was oxidized through three different stages and there was a transition stage where the phase transformed from suboxides to the equilibrium state of pentoxide. The applied stress and surface defects reduced the activation energy of oxidation.