Just wanted to share the new improvements regarding OAB in CU7, not yet released !
Exchange 2013 Cumulative Update 7 introduces the capability for an OAB generation mailbox to host a shadow copy of another OAB. This functionality enables additional Mailbox servers to be an endpoint for OAB download requests. By default, this feature is disabled and is configurable per OAB.
Ross has written about this in detail in :