An optical communication system, a transmission apparatus and a reception apparatus, and a transmission method and a reception method are provided. The transmission apparatus transmits an optical signal, and comprises: at least one encoder for encoding at least one data signal, respectively; at least one first modulator for performing first modulation on the at least one encoded data signal, respectively, and generating at least one first modulated signal; at least one second modulator for performing second modulation, which is DFT-spread-OFDM modulation, on the at least one first modulated signal, respectively, and generating at least one second modulated signal; and an up-converter for up-converting the at least one second modulated signal into the optical signal to be transmitted. Thus, a PAPR of the optical OFDM transmission signal is decreased, a signal distortion caused by non-linearity of optical devices is reduced, and a reception quality of the signal is improved.