As Dennis has said Eric, getting focus will likely be a problem without either a much lower profile focuser or shifting the mirror forward. Tornado33 uses one but he had his custom built.
I tried using an oag with my sct for autoguiding and gave up in frustration. I found it too difficult to find a guidestar most of the time, that I could use with the ToUcam. It might be different with the DSI with the longer exposure time.
Having said all that, there are a lot of great imagers out there who use an oag, I just don't know how they do it to themselves.
