Hello.
The OSC used to monitor the link and to send the communication signal from NE to NE .
The OSC separated from the main signal so in case a cut happened in a span . you can relize if the problem is in the fiber or in the boards , if the OSC still working while the main signal has cut so the problem is in the boards of the NE (specially amplifier) . But if the OSC and the traffic both cut so the problem is in the fiber .