Outlook 2011 (the Mac version is a year after Windows) is supposed to be scriptable, so it should be able to send items to Omnifocus. Outlook contains a mile long list of functionality and much of it is useful to a lot of people BUT: If Exchange is your only reason for choosing it, I would consider a few things first.
1. Snow Leopard and hence Mail, Address Book and iCal works well with Exchange 2007. It works less well with 2003, but your email is no problem (imap setup).
2. For Out of Office, you can do that from Outlook Web Access if that is set up for the Exchange domain. If you have access, you can also do it with remote desktop control
. In Exchange 2007 I think you can set up automatic intervals, like Mon-Fri 17-08:00 too, but your Exchange admin should know more about this.
3. If you work efficiently with what you have I see no reason to start looking into Outlook unless you have specific issues in your workflow that you would like to see improved. My personal conclusion is: MS Office is an industry standard, but 95% of its functionality is something that 95% of us do not really need, including most professionals. I still use Excel and Word (2008) but for practically all of my needs I could use Google's web tools, Open Office, etc instead. If you are to use MS Office professionally you should invest a lot of time in setting up your workflow around it to make the financial investment worthwhile. But why would you use it in the first place, if you use and like Omnifocus and it does what you need...?