Space-time block coding is a modulation scheme recently discovered for the transmit antenna diversity to combat effects in wireless fading channels. In this paper, using the equivalent single-input single-out (SISO) model, we present closed-form expressions for the exact symbol error rate (SER) and bit error rate (BER) of orthogonal space-time block codes (OSTBCs) with M-PSK and M-QAM modulations over flat uncorrelated Rayleigh/Nakagami- m fading channels. We also derive the exact SER of OSTBCs with M-PSK and M-QAM modulations for the case of correlated Rayleigh fading channels. Moreover, we show by several numerical examples the impact of antenna correlation on the performance of OSTBCs