There is some tolerance on stair height and depth. You could tack 3/8 or 1/4 luan to the risers and depending on height could probably do the same with the treads. Cover seams with trim. This would pass inspection unless the rise is already at the maximum height. But all the trouble of doing it would outweigh buying oak (or any other wood) stock and cutting your own treads. This would also allow the new riser wood to go below the tread depth. I think.
Where's atlanticwb? He would know for sure.
As long as the houses' price was inline with what they did, the new oak would raise the value of the home.