Optical performance monitoring is an indispensable feature for optical systems and networks. In this paper, we propose the concept of optical performance monitoring through channel estimation by receiver signal processing. We show that in coherent-optical-orthogonal-frequency-division- multiplexed (CO-OFDM) systems, critical optical system parameters including fiber chromatic dispersion, Q value, and optical signal-to-noise ratio (OSNR) can be accurately monitored without resorting to separate monitoring devices.