Based on a simple model of oxide film growth, we derive equations for oxygen tracer diffusion during the second stage in two-stage oxidation experiments. Using this model, we analyze tracer penetration profiles depending on temperature, time and the atomic mechanism of oxygen transport. In Part I the high-temperature A and the low-temperature C regimes of diffusion are considered. The results are presented as kinetic diagrams convenient for designing and interpreting experiments. Part II of this work will be devoted to the intermediate regime B.