One of the serious imperfections affecting orthogonal frequency-division multiplexing (OFDM) systems is transceiver in-phase/quadrature-phase (I/Q) imbalance. In this paper, the effect of I/Q imbalances on the transmit antenna selection (TAS) OFDM system is studied. The optimum antenna selection in presence of transmit and receive I/Q imbalances are proposed. We derive closed-form expressions for ergodic capacity of the system with transmit and receive I/Q imbalances. Moreover, some tight and simple lower and upper bounds for the ergodic capacity of the TAS OFDM system are derived. The simplicity of the proposed formula gives insight into the performance and optimization of the system. Finally, the analytical results are confirmed by simulations.