It's goddamn jazz(I mean every bit of it except for the Ending theme Calling, which is more pop, some minor big band influence here and there). How can you not recognize JAZZ when you hear it?! Unless you're like, 12 years old.
Also, to my knowledge, there's only the one OST and the Opening Single(Which contains both the instrumental AND the Vocal Opening which is pure sex.) http://rapidshare.com/files/111841896/_Nipponsei__Baccano__OP_Single_-_Gun_s___Roses__Paradise_Lunch