Waving hardwood floor
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&amp;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.