Exchange 2013 in coexistence scenario might trigger all clients to download OAB
Installing the first Exchange 2013 server in an organization that already has Exchange 2007 or 2010 might trigger all clients in the organization to download a new copy of the OAB, which can result in network saturation and server performance issues.
This occurs because Exchange 2013 creates a new default OAB in the organization that supersedes the Exchange 2007/2010 OAB. Mailboxes that do not have a specific OAB assigned to them, or that are located on a mailbox database that does not have a specific OAB assigned, will download the new default OAB.
To prevent this from happening, assign an OAB to every mailbox or database before installing the first Exchange 2013 server.