Recently, Space-Time Block Coded OFDM (ST-OFDM) that applies Space-Time Block Code (STBC) to OFDM has been proposed. Space-Frequency Block Coded OFDM (SF-OFDM) has been also proposed where the block codes are formed over the space and frequency domain. ST-OFDM and SF-OFDM are known as the schemes that achieve good performance over the multipath fading environments and the fast fading environments, respectively. For the systems with two transmit antennas, the orthogonal conditions required to separate the received signals are that in ST-OFDM, the frequency responses of the consecutive two OFDM symbols are almost identical and that in SF-OFDM, the frequency responses of the adjacent two subcarriers are almost identical. In practical fading environments, however, these conditions of the orthogonality sometimes cannot be satisfied. In those environments, the received signals cannot be well separated and the performances are degraded. Recently, the diagonalized maximum likelihood decoder (DMLD) of new zero-forcing (ZF) type was proposed for the space-time block coded single carrier QPSK system to maintain the orthogonality of STBC under the fast fading environments and the flat fading environments, where the channel separation in DMLD is performed by the ZF algorithm using two receive signals at time index 2n, 2n+1 (Space Time Code: STC) or two subcarriers (Space Frequency Code: SFC). Note that the matrix generated after the channel separation is not an identity matrix but the matrix proportional to an identity matrix. We show that ST/SF-OFDM with DMLD outperform ST/SF-OFDM in terms of Bit Error Rate (BER).