Firewall in attic

Can a firewall in an attic in a condo in Calif be OSB?
I thought it had to be 5/8 drywall.

Has to be 5/8 drywall here

There is such a thing as fire rated OSB, First I would check it that’s what has been used which I doubt and secondly I would check to see if it’s allowed in your area…

Normally dry wall , refer it to the local officials

http://www.nachi.org/forum/f18/osb-fire-rating-4999/

Osb is draft stop.