In over-the-horizon radar (OTHR) the need to preferentially select propagation mode arises when one or more modes are perturbed by ionospheric disturbances. Due to mixed-mode propagation and range-elevation coupling, such control is only implementable using noncausal beamforming via MIMO radar architectures. We introduce three key principles that govern mode-selective multiple-input multiple-output (MIMO) OTHR design. Numerical examples illustrate the high potential efficiency of mode-selective MIMO OTHR, while field trials support the introduced main principles.