In general it is possible to bring mobility assistance items into a MOIA just like usual items such as small suitcases. For safety reasons these items must be stowable in your seating area.
Items such as a rolling walker need to be folded for storage while on board. Please also note that there is a step entering the MOIA vehicle.
As the driver is responsible for all passengers' security throughout the trip with MOIA he will finally decide if you can take mobility assistance or other items on board.
Information about how people who require assistance can drive MOIA can be found here.