An OFDM system can suffer from two types of frequency error: firstly, a frequency offset (as might be caused by the tolerance of the local-oscillator (LO) frequency); secondly, an error in the receiver master clock frequency (which will cause the spacing of the demodulating carriers to differ from those transmitted, and the duration of the receiver integrate-and-dump process to differ from the reciprocal of the transmitted carrier spacing). The effects of both types of error are analysed. The analysis, applied to suitable synchronisation signals, could be used to help in the design of Automatic Frequency Control (AFC) strategies. It is also shown that, for symbols containing random data, either type of error causes inter-carrier interference (ICI). This effect is quantified as a function of the system parameters.