I have an existing floor that is 3/4 x 2-1/4 T&G oak nailed directly to the joists with cut nails – no subfloor. Will this be a sufficient base for a new hardwood strip floor? Will the oak subfloor be difficult to nail to? I could cover the existing floor with plywood, but only have room for a 3/8 thick plywood. Would this be recommended? Thanks!

You could install a new floor directly over the existing floor. However, you must run the new floor at 45 to 90 degrees to the existing floor strips. If you use a pneumatic nailer, you should not have a problem nailing into the old oak floor. Installing 3/8" plywood ( 1/2" is better ) over the old floor would allow you to install the new floor parallel to the existing floor.