This paper proposes a low-complexity coverage-oriented subcarriers assignment (COSA) algorithm which can efficiently assign the subchannels to enhance the coverage reliability of the orthogonal frequency division multiplexing (OFDM) based spatial multiplexing systems in the multiuser environment. Motivated by the increasing popularity of OFDM and the multi-input multi-output (MIMO) technique, how to use the multiuser and the frequency diversity simultaneously to improve the coverage reliability of the diversity-deficient multiplexing system is an essential issue. Because the total available transmit power is split uniformly across transmit antennas in the OFDM-based spatial multiplexing systems, increasing the number of transmit antennas leads to a smaller signal-to-noise ratio (SNR) per degree of freedom. Thus we propose the COSA algorithm to enhance the coverage performance of this system. Besides, by means of order statistics and Glivenko- Cantelli Theorem, we develop an analytical expression for the link outage probability and cell coverage reliability of the OFDM-based spatial multiplexing systems when using COSA, respectively.