In this paper, a hierarchical model of the optical transport network (OTN) with an optical supervisory channel (OSC) subsystem is discussed, and that the reliability of operation, administration and management (OAM) signals is rested with the quality of signals transported in OSC is indicated by analyzing the charac-teristics of signals in OSC. Basing on the one-to-one correspondence of Q-factor with bit-error-ratio (BER) in data communication systems, an on-line supervisory scheme for Q-factor of signals in OSC is presented, and the supervisory module by a digital signal processor (DSP) approach is designed to implement on-line supervision for Q-factor.