Orthogonal frequency-division multiplexing (OFDM) is an ideal transmission technique for dynamic spectrum access in cognitive radio (CR) systems. In this letter, we propose low-complexity spectrum shaping to enable fast decaying of power spectral sidelobes and enhance spectral compactness for OFDM-based CR systems. Based on a basic scheme mapping antipodal symbol pairs onto adjacent subcarriers, we present two modified schemes to further balance sidelobe suppression and system throughput. Compared with existing spectrum shaping schemes, ours exhibit their advantage of both simplicity and flexibility.