Carrier frequency offset (CFO) in OFDM systems, which induces the loss of orthogonality among OFDM subcarriers, can result in significant performance degradation. As a result, it needs to be estimated and compensated for. In this paper, we present a general CFO estimator based on the maximum likelihood (ML) estimation criterion, with which CFO can be obtained using training OFDM symbols, pilot tones, null subcarriers, or a combination of them. Using the proposed CFO estimator, the performance of CFO estimation can be significantly improved by taking advantage of the channel side information. In particular, using the channel statistics information, such performance improvement can be achieved for low SNR values and all SNR values over Rayleigh fading channels and Ricean fading channels, respectively. When the complete channel impulse response (CIR) information is available, simulation results will show that the performance improvement can be more than 6dB. To further demonstrate the capability of the proposed CFO estimator, we will consider an OFDM system using the signal structure of the IEEE WLAN standard 802.11a. Compared with previous work using null sub-carriers alone, we will show that by taking advantage of the pilot tones, null. sub-carriers, and channel statistics, the performance of CFO estimation can be improved by about 2dB.