I have a 1971 modular rancher with existing 3/4" oak. I have refinished several rooms but the dining room is in bad shape. I will be removing the old oak and replacing it with 3/4"x5" pre finished oak. The new floor will continue through the hall and into a new addition. The subfloors are at the same elevation in the addition and original house. My delemia is that the subfloor in the old house is 1/2" ply over metal framing. I can t nail the new floor to the metal framing and if I add more subfloor I will have a transition at the addition. Anyone have any suggestion I would appreciate them. Thanks

With one-half inch sub-floor the floor joists should be spaced at 16 inch centers, so there will be about 14 inches between supports for the sub-floor.
Instead of nailing your flooring 10-12 inches apart you should nail every 8 inches for the best installation.
!/2 inch sub-floors were common in the 70's and later. Your floor should perform well. I have seen 80 year old flooring nailed to the floor joists, no sub-floor, with a border!

Thanks for the response. I am still a little concerned about the holding ability of the staples in the 1/2" ply. The joist are 16" o.c. But there is some flex in the subfloor that I fear may cause the staples to loosen over time.

Staples hold the best. They should not penetrate into the body of the strips to cause splitting of the tongue, but just fit snugly to the edge of the strip.
If they penetrate too deeply you need to get a thicker "doughnut" or bumper for the gun. Staple every 6 inches if you like.