OAB stands for Offline Address Book... it is a file that is distributed to the clients either through Public Folders or a web share located on your Client Access Servers. Sometimes there may be issues with particular recipients... and when Exchange goes to update the Address Book, it fails for those users, thus they never get added.
To find out any users that are experiancing this issue bump the logging up on the following items in your Exchange Management Shell.
Set-EventLogLevel "MSExchangeSA\OAL Generator" -level expert
Set-EventLogLevel "MSExchangeSA\Proxy Generation" -level expert
Once these are on expert mode, perform a manual update of your offline address book:
Find out items in question under application log in event viewer.... repair the bad items. Ensure you return logging back to normal after.