This paper focuses on multiple-input multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) systems, where the MIMO channel order is larger than the length of the cyclic prefix (CP). By swapping the filtering operations of the MIMO channel and the fast Fourier transform (FFT), it is shown that each tone of a MIMO OFDM system can be viewed as a MIMO single-carrier (SC) system. As a result, the existing equalization approach for MIMO SC systems can be applied to each tone of MIMO OFDM systems. This so-called per-tone equalization (PTEQ) approach for MIMO OFDM systems is an attractive alternative for recently developed time-domain equalization (TEQ) approach for MIMO OFDM systems. The main difference between the PTEQ and TEQ approach is that a per-tone equalizer equalizes a single tone on the symbol level (low rate), whereas a time-domain equalizer equalizes all tones together on the sample level (high rate). Next to some other advantages, this means that a per-tone equalizer can much more easily be designed in practice than a time-domain equalizer. This is illustrated in the second part, where we adapt an existing semi-blind equalization algorithm for a generalized space-time block coded (GSTBC) MIMO SC system to a semi-blind per-tone equalization algorithm for a GSTBC MIMO OFDM system.