The answer is "oxidation involving Ta" in any case. If your film is thin and grained, oxidation may proceed on the grain boundaries... that is any path for carriers among different grains is closed. The micro-morphology and average thickness of the as-deposited film is crucial to understand the oxidation mechanism. By the way, if you do not need to anneal "in air" for other reasons, annealing should be done in inert environment (Ar, N2, vacuum).