I would address whatever caused the subfloor to sag that much in the first place. OSB decking in a humid area will have all sorts of problems, especially crawlspaces with backed fiberglass pinned up against them. All wood, even engineered woods like OSB and plywood, lose a lot of strength when they've swelled up due to moisture issues.
I had a similar situation this last year and was able to address it by using well-placed screws from below. That was 5" wide T&G, though. obviously you're flying blind from below, so you have to guess where the center of the boards are. With hardwoods (and especially narrow 2" seasoned white oak) I would fear splitting the board if you were either too close to the edge or end. Oh yeah - and this job is awful to do while in a crawlspace. I mean awful awful. Like "I am never doing this again IF I survive this ordeal" type of awful.
I didn't finish one room, so it still squeaks when you walk over it. However, that squeak doesn't seem to bother me nearly as much as it did before I decided it was a good idea to take 5 years off of my life by doing that type of work in that inhospitable location.