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In this letter, we calculate an exact closed-form expression for the bit error rate (BER) of orthogonal frequency division multiplexing (OFDM) systems with direct conversion that employ maximal ratio combining (MRC) reception in multipath Rayleigh fading channels, using binary phase-shitf keying (BPSK) modulation. We assume a realistic system model where direct current (DC) offset, carrier frequency offset (CFO) and imperfect channel state information (ICSI) are simultaneously considered. Results show the appearance of an irreducible BER floor due to DC offset and ICSI. As a rule of thumb, we provide a simple expression for the maximum DC offset allowable in a direct conversion receiver.