I'm a bit confused. You say: "What I would like to do is have our 15 mac users be able to view only a shared calendar from outlook."
First, there is no way I know of for them to see only a delegated calendar in the Calendar app (no longer called iCal in 10.8, FYI). They have to have their own primary calendar, then they can see a delegated calendar from a specific user below the primary one and will see that shared caledar based on whatever permission they have.
The question is: does each Mac users have their own Exchange account?
If so, you should be able to share the calendar you want to share from the sharing user's Outlook 2010 by right clicking the calendar in question and giving any of a variety of rights to any user under the domain. You have to search for the user you want to delegate to and add them, but you can definitely add them. (I am speaking based on my experience with Outlook 2011, which is considered an inferior product compared to Outlook 2010, so 2010 should be just as capable of this if not more. You could also use a similar process using Outlook Web Access [a.k.a. OWA] online i.e. webmail.)
If they don't have their own account and you are trying to use just one Exchange account for all 15 Mac users (which is inviting headaches, IMHO), it should still be possible, but only to do what I said above i.e. they have a primary calendar, then see a delegated calendar with whatever permissions you specified.
Again, I 100% know this can work using Outlook for Mac 2011. It's very hard to believe Outlook 2010 cannot do the same thing. And you can always resort to OWA.
If you think you've done all this right and it still isn't working in the Calendar app, I'd suggest trying your setup in a trail verison of BusyCal. The developer just added Exchange support and it's a great product. You can download a 30 day trial from the website. If everthing works, you know it's a Calendar app issue and you can call Apple for support. If it doesn't, then you either have a problem on the configuration side (your problem) or Office 365 isn't working like it's supposed to (Microsoft's problem - reach out to support).
Good luck!