Before I got Lasik, I almost always modified goggles to fit over my glasses. It seems like OTG goggles are never as nice as regular goggles.
I would take an exacto knife and cut a 1/4" groove on the side frame so there would be ample room to fit over the temples of my glasses. On colder days, I found that it could get pretty drafty and cause some tearing, so I would try to take some soft silicone and try to glue the missing piece of foam back onti the opening. This never worked & I eventually quit doing it.
I also tried to cut just a slit into the frame, but it giggled my glass frames too much. The 1/4" groove worked best.
My glasses were always smaller wire-framed models, so they fit into the goggles pretty well. With medium to large volume goggles- like the Smith Prodigy- I think you should be able to fit a pretty large pair of glasses under there.