Probably the same type of acrylic primer that is typically used for traditional oils. That would be one that exhibits a degree of "tooth", so that the oil paint can physically bond with it. My choice of acrylic primer is Grumbacher 525 Acrylic Gesso. I thin it to the consistency of cream, and I brush it on with a household sash brush.
While I have my own opinions, those who recommend the use of WMO's claim them to be "the same as traditional oil paint". Well, of course that can't be totally true, or they wouldn't be sold as "Water Miscible Oil Paints". By pure logic, there HAS to be SOME differences, doesn't there?