You may have seen this when you use Exchange 2013 ECP to create a mailbox and want to select a specific OU. Then for what it looks the OU picker don’t show all OUs, only some of them are shown.
Reason is that the ECP webapplication has a limit of not showing more than 500 OUs, rest of them wont simply be shown.
Don’t cry, there is a solution if you have some CPU cycles to spare. Edit the “web.config” in the ECP directory. Default path is “C:\Program Files\Microsoft\Exchange Server\V15\ClientAccess\ecp\”.
In the appsettings section, add the XML element
<add key="GetListDefaultResultSize" value="5000" /> where the 5000 is number of OUs you want to be shown.
When done recycle the MSExchangeECPAppPool to make ecp webapplication read the configuration and have it live and kicking.