This study presents a systematic approach to monitor sustained oscillations in real-time power systems. In general, monitoring of sustained oscillation means detection of oscillating frequency, its time range, type, and source of sustained oscillation. The basic steps adopted in paper include double-stage mode decomposition (DSMD), spectral and statistical analysis. The DSMD follows decomposition of signals by using improved complete ensemble empirical mode decomposition with adaptive noise followed by further decomposition by using variational mode decomposition to obtain rich dominant mode. It is shown that decomposed mode obtained by DSMD carries monotonic frequency component. The proposed approach is validated on simulated signals and real-time phasor measurement unit signals. The frequency and time duration of interarea and forced oscillations are distinctly detected. Also, the source of forced oscillations is identified in study. The proposed approach is more effective and accurate in wide-area monitoring than the other spectral-based methods for oscillation analysis in phasor measurement unit signals.