Power oscillation is a serious concern for power system security. Locating the oscillation sources online can help take specific measures to attenuate the oscillation. Existing methods can successfully determine the region or generator that the oscillation source exists， but how to locate the source in the control devices of the generator has not been solved. Based on the Hamilton realization， the internal energy structure of the generator is analyzed， and the energy injected by the generator is decomposed into two components corresponding to the governor and the exciter， respectively. On that basis， a method of distinguishing the control device where the oscillation source exists is proposed. Simulation results on a single-machine infinite-bus system and IEEE 39-bus system prove the correctness and validity of the method. All measurements are from the local bus online， so this method can not only be applied in control centers， but also can be used in the stations for distributed monitor.