Hardwood over hardwood?
i will have to argue that and say yes. as long as the new floor will be perpendicular to the old oak floor. if not then you can either add thin plywood over everything, or remove the oak and install 3/4 plywood. in my house i have an oak floor that was laid directly over my pine tounge and groove subfloor, at least 50 years ago. put it this way, when we refinished it 3 years ago it was the last time it could be sanded. it is still nice and tight and flat. hell about a 4' circle of it was water damadged when i bought the house. swelled and buckled up. i let it dry out and most of it went back down, i had to replace maybe 1'x2' of that section. is the pine sub floor considered hardwood? no, but it is tounge and groove boards, much like hardwood. there will be the same type of expanision and contraction. in the upstairs of my house i put a rosewood floor over 11/32 plywood over my pine subfloor. i used the plywood as a base as i wanted to run my rosewood the same direction as the pine tounge and groove. if i wanted to run it perpendicular to the pine i would have not used the plywood. my rosewood floor is flat like glass. when iinstall my new floor dowstairs im going right over the old oak floor that has been sanded to roughly 3/8" and it will be ok. i used to do hardwood floors and my best friend still does them, owns his own wood floor business. we have both been on refinish jobs that were hardwood over hardwood, over subfloor. they were just as tight as any floor that was over plywood. or any other solid subfloor. use a good amount of staples and you should be fine. is it the best way to do it, no, but it can work.