With a plan-view transmission electron microscopy technique of directly imaging buried Si/SiO2 interfaces, we studied the interface roughness resulting from the oxidation process. Our results show that thermal annealing in nitrogen at 900 degrees C can dramatically remove the interface roughness for Si(100)/SiO2 interfaces [X. Chen and J. M. Gibson, Appl. Phys. Lett. 70, 1462 (1997)]. In contrast, Si(111)/SiO2 interfaces,which tend to be smoother than Si(100)/SiO2 interfaces, are not affected by annealing. A model to link interface roughness and silicon oxidation kinetics was developed. This model not only qualitatively explains the difference between Si(111) and Si(100) interfaces that we saw but also shows that oxidation dynamics is the origin of the interface roughness. Hence, it might be a new approach to understand oxidation dynamics through studying interface roughness.