1. Check if OAB exists (Exchange 2007 Management console)
2. Check if user configured to use expected OAB (Get-Mailbox)
3. Try to re-build OAB (Exchange 2007 Management console, safe procedure from HMC stand point). Enable OALGen logging to see if any errors prevent OAB from being generated.
4. Check HmOabUpdate.exe log to see if it works.
5. Check Outlook version and OAB distribution mechanism. If you use only web distribution for OABs (no PF distribution), Outlook 2003 (and earlier) can not use it.