This paper proposes an open-loop correlation reduction precoding scheme for overloaded multiple-input multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) systems. In overloaded MIMO-OFDM systems, frequency diversity through joint maximum likelihood (ML) decoding suppresses performance degradation owing to spatial signal multiplexing. However, on a line-of-sight (LOS) channel, a channel matrix may have a large correlation between coded symbols transmitted on separate subcarriers. The correlation reduces the frequency diversity gain and deteriorates the signal separation capability. Thus, in the proposed scheme, open-loop precoding is employed at the transmitter of an overloaded MIMO system in order to reduce the correlation between codewords transmitted on different signal streams. The proposed precoding scheme changes the amplitude as well as the phase of the coded symbols transmitted on different subcarriers. Numerical results obtained through computer simulation show that the proposed scheme improves the bit error rate performance on Rician channels. It is also shown that the proposed scheme greatly suppresses the performance degradation on an independent Rayleigh fading channel even though the amplitude of the coded symbols varies.