What is the difference between ISI and MAI in MIMO system?
MAI is when there is Interference due to multiple users accessing a common wireless media. So if you consider an OFDM system then as long as users are trasmiting in their allocated sub carriers there is no multi access interference irrespective of the MIMO technique used, delay spread etc.

If each user is using the same frequencies to transmit simultaneously, then there is Interference due to multiple access, this interference will happen irrespective of multipath, placement of antennas.

In case of Space division multiple access the it is theoretically possible to allocate precoding matrices to users such that they don't interfere with each others data streams while utilizing the same frequency resources. But this is only possible after suitable post processing at the receiver to separate out the data.

So anytime non orthogonal resources (time, frequency, codes, space) are used by users simultaneously there will be multi user/access interference, irrespective of multipath, delay spread, antenna placement.