The Exchange 2007 File Distribution Service is responsible for keeping the OAB on the CAS in sync with the mailbox server. The default setting only polls the OAB every 8 hours. This could mean that a change you make doesn’t make it to the Outlook clients until well over a day (if you leave the OAB creation to the default once a day)!
To change this setting to e.g. every hour (60 minutes) use this command:
Get-OabVirtualDirectory| Set-OabVirtualDirectory -pollinterval 60