I have kerfed the back of maple for a similar application and had good results. The problem with oak is that it tends to kink where the board may have a cup. . This will happen when one of the kerf cuts is deeper or shallower because of the cup. It, the oak, is also more brittle and seems to crack more easily than the maple
We have considered using plywood (3/4" oak), but the total length of the riser is longer than 8'. We can get oak veneer 10' long to apply to a pine board. The problem we fear is that the pine may "move" too much and the veneer will work loose or buckle in some fashion.
I may be all wet about the movement of the softwood substrate though....
Oak plywood would actually work great, but again, there are no 10' sheets available in our area.
OH well, if this were an easy problem, I wouldn't have to ask the experts in this newsgroup.
Neil