Is it possible for multiple users to simultaneously transmit on the same subcarrier in a MIMO-OFDMA system?
For a given OFDM symbol, no subcarrier can be assigned to two users simultaneously. Each subcarrier is uniquely assigned to a unique user. Else you have to implement an Interference cancellation at receiver (which would require having channel knowledge of other users as well, meaning global CSI) or at the transmitter and extremely complex interference alignment/precoding has to be done.
In practical systems like LTE or 5G, thus one subcarrier in a given OFDM symbol is allocated to only one user for the reasons listed above.