We use ISO 10011 to audit ISO 9000. What standard do they use in auditing OHSAS 18001?

You use ISO 10011 to help establish your audit programs and develop your auditors, it is a guidance document. You need to get a copy of ISO 19011 and start using it..

For OHSAS you can use anything you want to help develop your audit program, but you audit against the "standard".