If the OSB has been screwed down (rather than nailed), you can adhere directly to it. Your membrane will likely last longer with recovery board, though. Plywood can de-laminate, so it's not a good choice for use w/o recovery board.
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Both are ok to glue to if screwed as said before. It is however better to use a recovery board to protect the EPDM. Also it takes more glue to glue to plywood and even more to OSB then a recovery board.