As I pull the old carpet upstairs to install some doug fir flooring, I have decided to go ahead and take some Kilz and prime/seal the surface of the OSB subfloor, just to help prevent rot from any moisture that may seep through from a spilled drink, and perhaps add a little to the felt barrier to prevent the possibility of mold, etc. I plan on using regular 15# roofing felt, since this floor is actually a ground floor above a daylight basement. Beneath the sub floor is a heavy layer of insulation vatting, and the drywall below that which is our basement ceiling. I've never had any issue with excessive moisture, but just wondered of the primer for the OSB sub might be overkill?

It is overkill. If moisture gets down to the primer it will take a long time to dry out.