In this paper, a pilot-tone-based maximum likelihood estimator (PTB-MLE) for carrier frequency offset (CFO) in orthogonal frequency division multiplexing (OFDM) systems is proposed. To obtain consistent estimate of the CFO over frequency-selective fading channels, the proposed method employs a preamble comprising distinctively-spaced pilot tones. As a result of this preamble configuration, a large estimation range equal to the bandwidth of OFDM signal can be achieved. Different from previous PTB estimators that are ad hoc, the PTB-MLE exploits the relationship between the CFO and the periodogram of the preamble. To realize this PTB-MLE in practice, a sub-optimal estimator based on zero-padded FFT is introduced as well. When the oversize ratio is sufficiently high, the sub-optimal estimator is asymptotically unbiased and its variance approaches the CRB.