In this paper, I propose a novel orthogonal space-time-frequency block code for orthogonal frequency division multiplexing (STFBCOFDM) systems using Alamouti scheme with two transmit antennas. In the proposed code, Alamouti code is applied over two successive OFDM symbols and neighboring subcarriers simultaneously with a scaling factor in order to enhance the system performance over time varying or frequency selective fading channel. The performance of the proposed system is also analyzed in spatially uncorrelated time-varying multipath Rayleigh-fading channels. The analytic results involve a single finite-range integral, which can be easily computed numerically. The analytic results, which closely matches the numerical results, show that the proposed system outperforms STBC-OFDM in time-varying channels and SFBC-OFDM in frequency-selective fading channels.