In-phase/Quadrature-phase (IQ) imbalance can result in severe performance degradation in optical Direct-detection Orthogonal-frequency-division-multiplexing (DD-OFDM) systems. We build two optical back-to-back DD-OFDM systems, which implement Double-sideband (DSB) and Single-sideband (SSB) modulation, respectively. The tolerance to IQ imbalance of these two systems is analyzed and compared by using Error Vector Magnitude (EVM) and Symbol Error Rate (SER). It is found that, in the back-to-back case, the DSB system has stronger robustness to IQ imbalance than the SSB System. We further build two optical DD-OFDM systems each with 40-km transmission, which respectively implement DSB and SSB transmission. Similarly, the tolerance to IQ imbalance of these two systems are analyzed and compared in terms of EVM and SER. It is found that, however, in the case of 40-km transmission, the SSB system has stronger robustness to IQ imbalance than the DSB system. As a result, it is concluded that, in the case of transmission, SSB modulation can enhance the tolerance to IQ imbalance of DD-OFDM systems.