I'm prepping a 2nd flr 3/4 ply subloor and I just put a ton of screws into it. Probably due to defects in the joists, I have a few dips that follow the joists for 4-6 ft in length, about 8" wide and 1/8" deep. I figure this is a problem since I'll be laying 3/4 x 2 1/4 red oak perpendicular to the joists. Rather than sanding the floor around the dips or using filler, would it be ok to try and fill the dips with extra layers of 15lb felt?

The weird thing is that the nailing pattern is 12" oc. On the 1st floor, I can see the joists are 16" oc from the basement. Why do you think the builder used 12" oc joists on the 2nd floor?

Thanks!

That 12" center on a second floor is pretty common. It is because of the span and where the load bearing walls are situated.

Fill in those low areas with either 15 or 30lb felt feathered out. 90lb roll roofing or standard 3-tab roofing shingles will also work for that.