If you are using google apps and Outlook there is no way of configuring an out of office message in Outlook. This is annoying and it will require you to login to your gmail account in the browser to set the Out of Office message. To login go to your gmail use the url http://mail.google.com/a/yourdomainname.com. You will need to change yourdomainname.com to your domain name.
Once the setting page has opened scroll down to the “Out of Office AutoReply” section and select the “Out of Office Autoreply on” radio button and complete the necessary fields and scroll down the page and click on the save settings button