[EX2010] Upgrade Order

You should upgrade your Exchange 2010 server roles in the following order:

• Client Access
• Hub Transport
• Unified Messaging
• Mailbox
• Edge Transport

From <https://technet.microsoft.com/en-us/library/bb629560(v=exchg.141).aspx>

 Upgrade your Exchange 2013 server roles in the required order. First, upgrade Mailbox servers and then upgrade Client Access servers.

From <http://blogs.technet.com/b/justin_gao/archive/2013/04/09/you-need-to-know-how-to-apply-exchange-server-2013-rtm-to-cumulative-update-1.aspx>

Therefore, it is important to upgrade your Client Access servers prior to upgrading your Mailbox servers if you have multiple OAB generation mailboxes deployed in your environment.

From <http://blogs.technet.com/b/exchange/archive/2014/05/13/oab-improvements-in-exchange-2013-cumulative-update-5.aspx>