In the old days when using Exchange 2007 for hosting scenarios you would use the Configuring virtual organizations and address list segregation in Exchange 2007 whitepaper. In Exchange 2007 the msExchQueryBaseDN property on a mailbox was used to limit the search scope of users in OWA. The typical setting of this property is the OU where the users would reside in Active Directory.
The msExchUseOAB property on a mailbox is used to select an Offline Address Book in a hosting environment (where multiple OAB exist of course). This way the user would receive the OAB of his particular organization.