Similarity and difference between orthogonal frequency division multiplexing passive optical network (OFDM-PON) and wavelength division multiplexing passive optical network (WDM-PON) are dicussed, indicating that WDM-PON is actually a kind of physical layer technique which can perform the functions of data transmission system and network only by combining itself with some media access control (MAC) technique. In contrast, OFDM-PON is an optical access network working with the technique of orthogonal frequency division multiple accesses. Even its optical network units (ONUs) occupy different optical spectra, it is not a WDM-PON. Concept of OFDM-PON is clarified from two viewpoints: the optical beat interference (OBI) and MAC, to promote the development and standardization of OFDM-PON.