OSB (oriented strand board) makes for a great, and cost effective, roof deck, but plywood is substantially stronger.<quoted text>
No rebar. Had some screen and was sectioned with metal flashing. Was a lightweight concrete of some sort. If they hit it just right they could scoop a couple of feet sheets with a roof rake. Which wasn't all that often. Was 1-1/4 thick. Those guys worked.
This was laid on 7/16 sheath. Took a permanent sag. Just laid another layer of sheath on top instead of removing. Was in very good shape except for that sag and a lot cheaper than tearing off. Was 20 years old.
I'm surprised they didn't tear off your old roof completely. Was that part of the contract or did they just not wanna do it?