Im installing 3/4 x 3.5 inch hardwood upstairs, stapled.
The floor joist are 24 center.
The current Subfloor is 3/4 OSB.

The guys at Lowes told me I would need to pull the sub up and replace it with 3/4 T&G Plywood.
They said that it would void the warranty if I didn't
I'm not too concerned with the warranty, since most of them are a joke.

The real question is, will there be any real problems with the install.
Is the OSB enough for 24inch spacing and hardwoods?
There is a little flex when walking on the OSB now.

Im doing a 12x16 room and hallways. I imagine the halls are ok with OSB but is the main room?
Or does it all need to be replaced?

thanks in advance for you advice.

I would sheet out over the existing OSB with 1/2 CDX plywood. No reason for removal. Stronger subfloor not removing the existing OSB, and no need for a t&g plywood.

Yes, as stated, overlay the OSB with ½" plywood, using 1" screws on 8" centers carpenters glue optional(jerry liked to do this) 2" screws into the floor trusses. Overlapping all the seams in the OSB so the don't line up with the plywood seams.