Assuming floor joists are properly(sufficiently) sized I would put down 1/2" plywood over the osb. Shoot down some nails/staples in osb first to eliminate potential squeaks. Stagger layout of osb to plywood as efficiently as possible. Use as much construction adhesive as you want to buy and/or haul in to bind it all together.
As for insulation:
I've seen some DIY/semi-contractor two part spray foam kits at AIH. Might be worth investigating. ...cost? Open or closed cell?...
Other option would be to add mineral wool or dense fiberglass bats(unfaced). Follow up with stapling up a house wrap (breathable vapor barrier) to the bottom side of the joist.
Last edited by JimInAK; 04/16/21.
