Sawtooth oscillations appear to be the result of a cyclic process in which the plasma core is resistively heated until the safety factor drops below unity, causing the m = 1 tearing mode to become unstable and ultimately to flatten the electron temperature and safety factor profiles. A systematic model based on this hypothesis is shown to provide a consistent interpretation of ORMAK experimental data, including for the first time an explanation of the accelerating growth rate of the m = 1 precursor oscillations and the repetition time of the sawteeth.