—In this paper we present an analytical approach to evaluate the M-QAM bit error rate (BER) as well as channel capacity of Alamouti space time coded OFDM systems in case of direct conversion receivers subject to the simultaneous effects of carrier frequency offset (CFO), channel estimation errors, outdated channel state information and flat receiver I/Q imbalance in Rayleigh time and frequency selective fading channels. The BER and channel capacity is numerically evaluated using a closed form probability density function that is derived in the paper.