You don't say what version of OpenAudit you have but I'd first make sure you're on the latest version from
SVN.
Then as OAv1 is no longer maintained you might try my admin_nmap_input.php file to see if it helps. I'm not sure my latest changes are anything better than what's in the SVN trunk.
Also I think the nmap input code doesn't handle nmap scans that were done across subnets and thus don't have a MAC address in the output. It's best to run the nmap scan against machines on the same subnet as the scanning machine.