An optical add-drop multiplexer (OADM) for adding and dropping wavelength groups for each node among multiplexed and input wavelength groups includes: a wavelength group demultiplexer for separating a plurality of input channels into groups of channels and dropping a channel group required for the node and passing the other groups of channels therethrough; a channel selector for selecting channels from the wavelength group output from the wavelength group demultiplexer and transmitting the selected channels to the area; a channel multiplexer for multiplexing the channels having the same wavelengths as those of the selected channels and input from the area into the wavelength group, and a wavelength group multiplexer for adding the wavelength group input from the channel multiplexer to the wavelength groups that have passed through the wavelength group demultiplexer. Wavelengths required for each node are set to a wavelength group, thereby reducing the time necessary in extending channel capacity or wavelengths in each node. Also, since only a channel selector is additionally installed in an OADM for wavelength extension, wavelength extension is facilitated and a WDM system can be constructed at a low cost. Such an OADM may be used in a wavelength division multiplexing (WDM) optical link.