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.