I dunno about air, but the heat from fires doesn't penetrate very far underground, thus the reasoning behind underground bunkers.
With the speed these fires were going, the bunkers would have had oxygen from vents most of the time, and only a short while without it while the fire burned overhead. I would think there would be more than enough oxygen trapped in the bunker to allow the occupants to breathe while the fire passed.
'Course this is only guessing, but it seems reasonable enough to me.
Also if the idea of community fire bunkers do go ahead, I'm sure they will put in oxygen recycling or storage facilities in there.
EDIT: Oh and the vents can be closed, preventing toxic gasses from seeping in while the fire is overhead.