From the Exchange Shell:
get-oabvirtualdirectory | format-list > c:\oab.txt
Open the oab.txt file. The Name field will be used in the “Identity” command below. The Internal and External URL will be used in the “InternalURL” and “ExternalURL” below.
Remove-OabVirtualDirectory -Identity "OAB (Default Web Site)"
A Virtual Directory named “OAB” must not exist in IIS, if it does it will need to be deleted before running this command.
New-OABVirtualDirectory -ExternalUrl http://mail.domain.org/oab -InternalUrl http://mail.domain.org/oab