Cognitive radio (CR) technology provides great flexibility in spectrum utilization with orthogonal frequency-division multiplexing (OFDM) as its candidate transmission technique. In this paper, we propose a simple spectrum shaping scheme for OFDM-based CRs to enhance spectral compactness and ensure bandwidth efficiency. By mapping antipodal symbol pairs onto adjacent subcarriers at the edges of the utilized spectrum band, our scheme enables fast power spectral sidelobe decaying without bringing much extra complexity to the transmitter or receiver. Sidelobe suppression and system throughput can be well balanced by adjusting the coding rate while power control on different sets of subcarriers will further deepen the sidelobes. The proposed scheme is also validated by our simulation.