This paper presents a complete overview of low-frequency oscillation phenomena in power systems. The definition of low-frequency oscillation and its classification are clearly explained. Some power system blackout incidents due to low-frequency oscillation and the lesson learned from those incidents are also reported in the paper. Widely used methodology for studying low-frequency oscillation among power utilities is presented. Methods for oscillation damping, at both the operational and the planning stages of a power system, are briefly discussed. A comprehensive case study of low-frequency oscillation in a simple system is presented using eigenvalue analysis. Time domain results are presented to support the eigenvalue analysis. Conclusions are duly drawn.