The current subfloor is 3/4" osb and is in ok condition. I read
someplace that to install the flooring parallel to the joists an extra
1/2" plywood layer would be a good idea. Is this necessary or is is
overkill?

OSB is an unsuitable substrate for hardwood floor; doesn't hold the
nails. An additional layer of plywoood is not only a good idea but
necessary. Screwed and glued. 3/4" would be better in your
application. Check the osb for flatness first.

Mike