A modern LCD monitor should be able to display the DOS prompt, a TV not so much though. Have you tried one of the old Gefen VGA to DVI scalers by any chance?
You can rest assured that the OSSC supports all kinds of things, but the big question is will your display support it? One thing the OSSC cannot do is frame-rate conversion. Some old DOS games ran in 70hz, most TVs will just reject that unless it’s frame converted to 60hz.