PDQ wrote:

3/4" means that one can walk on the roof without that vague feeling of being
asea.

Wander about on your roof much? I've seen 300 pound guys on 1/2"
ply on 16" centers. They didn't fall through.

I have the 5/8 on my roof (code is really the minimum acceptable) and it sinks
beneath my feet.

Deflection is expected with load. If the sheathing was made up of 2x
material it would still deflect. There's the balance (especially
important on a roof!) one tries to achieve between structural strength
and cost. Adding more weight, and cost, doesn't necessarily make it a
better structure.

OSB vs. plywood is 6 of one and half a dozen of the other. It just depends on how
much one is willing to pay for roofing.

Exactly.

R