Optical amplifier apparatus (100, 200) comprising: an input for receiving an incoming optical signal comprising an optical supervisory channel, OSC, signal, the OSC signal comprising an out of band, OOB, OSC signal at first wavelength or an in-band, IB, OSC signal at a second wavelength where the first and the second wavelengths are different, and a plurality of service channel signals at channel wavelengths of a wavelength division multiplexing, WDM, channel wavelength grid; an output for outputting an outgoing optical signal; an optical amplifier (106, 206) having an input, an output and an operating bandwidth that includes the WDM channel wavelength grid; a first optical drop filter (104, 204) provided upstream of the optical amplifier input, the first optical drop filter configured to drop from an incoming optical signal the out-of-band, OOB, OSC signal at a wavelength outside the optical amplifier operating bandwidth; a second optical drop filter (108, 208) provided downstream of the optical amplifier output, the second optical drop filter configured to drop from an incoming optical signal the in-band, IB, OSC signal at a wavelength within the optical amplifier operating bandwidth; an OSC signal output configured to output the OSC signal dropped from the first optical drop filter or the second optical drop filter; an OSC signal input configured to receive an outgoing OOB OSC or IB OSC signal; and an optical add filter (110, 210) provided upstream of the output, the optical add filter configured to add the outgoing OOB OSC or IB OSC signal to a plurality of service channel signals at channel wavelengths of the WDM channel wavelength grid to form the outgoing optical signal. an input for receiving an incoming optical signal comprising an optical supervisory channel, OSC, signal, the OSC signal comprising an out of band, OOB, OSC signal at first wavelength or an in-band, IB, OSC signal at a second wavelength where the first and the second wavelengths are different, and a plurality of service channel signals at channel wavelengths of a wavelength division multiplexing, WDM, channel wavelength grid; an output for outputting an outgoing optical signal; an optical amplifier (106, 206) having an input, an output and an operating bandwidth that includes the WDM channel wavelength grid; a first optical drop filter (104, 204) provided upstream of the optical amplifier input, the first optical drop filter configured to drop from an incoming optical signal the out-of-band, OOB, OSC signal at a wavelength outside the optical amplifier operating bandwidth; a second optical drop filter (108, 208) provided downstream of the optical amplifier output, the second optical drop filter configured to drop from an incoming optical signal the in-band, IB, OSC signal at a wavelength within the optical amplifier operating bandwidth; an OSC signal output configured to output the OSC signal dropped from the first optical drop filter or the second optical drop filter; an OSC signal input configured to receive an outgoing OOB OSC or IB OSC signal; and an optical add filter (110, 210) provided upstream of the output, the optical add filter configured to add the outgoing OOB OSC or IB OSC signal to a plurality of service channel signals at channel wavelengths of the WDM channel wavelength grid to form the outgoing optical signal.