Multipath ionospheric propagation (MIP), a typical phenomenon the sky wave over-the-horizon (OTH) radar may encounter, is often viewed as an annoying factor to be eliminated in the traditional OTH radars. In this paper, we consider multiple-input multiple-output sky wave over-the-horizon (MIMO-OTH) radar with closely spaced antennas. We develop a signal model using the multi-quasi-parabolic (MQP) ionospheric model, based on which we derive an optimum detector according to the Neyman-Pearson criterion. We show that MIMO-OTH radar has superior detection performance over the conventional OTH radar and that the existence of MIP can improve the detection performance of the MIMO-OTH radar system.