The present invention provides a method and system for monitoring composite optical signals carried over plural fiber-optic lines within an optical network. Methods and systems in accordance with the present invention each utilize a single OPM to monitor each one of a set of sample proportions of composite signals split off from respective fiber-optic lines. In one embodiment, an optical performance monitoring system comprises a plurality of fiber-optic lines, each fiber-optic line carrying a respective composite optical signal, a plurality of optical taps, each optical tap being optically coupled to a respective one of the fiber-optic lines and splitting a portion of the composite optical signal thereof, a plurality of optical switches, each optical switch being optically coupled to a respective one of the optical taps and comprising either an Open configuration and a Closed configuration, an optical coupler, and an Optical Performance Monitor (OPM).