Multi-user Orthogonal Frequency Division Multiplexing (OFDM) have been widely adopted to combat the detrimental effects of wireless channels and enhance system throughput. Recently, interference alignment is proposed to exploit interference to enable concurrent transmissions of multiple signals. In this paper, the authors investigate how to incorporate interference alignment in multi-user OFDM systems. They first reveal the unique characteristics and challenges brought about by using interference alignment in diagonal channels. They then derive a performance bound for the multi-user OFDM/interference alignment system under practical constraints (i.e., a finite number of subcarriers), and show how to achieve this bound with a decomposition approach. The superior performance of the proposed scheme is validated with simulations.