The current state of the synchrophasor measurement (SPM) technology opens up new possibilities for analyzing the operating modes of power systems. One of the most important applications of processing the SPM data is monitoring of the power system oscillatory stability. It includes the analysis of low-frequency oscillations (LFO). The problem of detecting the source of LFO is one of the most urgent. This paper considers the methods for detecting the LFO source. One of them estimates the quantity and direction of the flow of oscillation energy. Another one compares the amplitude-phase characteristics of the processed signals. Examples of real LFO in the Russian power system, given in the paper, illustrate various aspects of the analysis. The research proposes ways to improve the methods of analysis of the LFO based on the ideas about the structure of the phasor measurement units signals... Presentation