The OzHosting namserver addresses are:
nsa.ozhosting.com
nsb.ozhosting.com
nsc.ozhosting.com
If your domain is NOT registered within your OzHosting account you can still manage the DNS records for your domain via your Ozhosting Control Panel by pointing the nameservers to Ozhosting by using the nameserver records above.
You will need to login to the control panel where your domain is registered and update the nameserver records as per above.
After you have updated the namserver records with the provider from whom you have bought the domain you then need to activate DNS management within your Ozhosting Control Panel - this article tells you how to do that: