In the presence of different IQ imbalance (IQI) levels in the different nodes of a cooperative system, it becomes necessary to investigate the effect of each imbalance parameter on the performance. In this contribution, we investigate the outage performance of OFDM based Decode and Forward (DF) and Amplify and Forward (AF) systems over Rayleigh channels under IQI. In particular, approximate outage probability expressions are derived and the effect of different IQI parameters is analyzed for different ranges of SNR. The diversity order is also derived depending on the IQI levels. Furthermore, we explain when and where it is more beneficial to invest in compensating the imbalance or in increasing the transmission power. Based on the outage performance, a comparative study between AF, DF and direct link transmission techniques is also discussed depending on the IQI parameters, the SNR range, the transmission rate and the relays position.