I have removed the old hardwood from my living room and found 3/4" x 12" pine planks for the sub-floor. I've located all the squeaks and will be screwing fasteners into those locations as well as replace the bracing below. Once that's complete I'll be adding new felt over the planks and installing 3/4" osb over that. Do you think adding the layer of osb is necessary or overkill? If it is not overkill what type of fastener should I use on the osb? #8D shank, Glue and screw with 2.5" or 3" screws?

Lastly, there is bracing that I won't be able to reach from below. Heat duct is in the way. Should I take the time to remove the planks in those areas to replace the bracing below and re-install planks and proceed?

Thanks in advance for you're adivce

J.