The EASIEST (not necessarily the most elegant) way is to access the mailbox via Outlook Web Access and set the OOA message in the "Options" section of the mailbox. This is the only way to access the OOA from a Macintosh client (to my experience) and the most efficient way for a shared mailbox. Alternatively, the mailbox owner (the primary user) can set the OOA if logged in to the mailbox using Outlook under "Tools => Out of Office Assistant", but you need to know who owns the mailbox, and in some organizations, that is hard to determine.

The EASIEST (not necessarily the most elegant) way is to access the mailbox via Outlook Web Access and set the OOA message in the "Options" section of the mailbox. This is the only way to access the OOA from a Macintosh client (to my experience) and the most efficient way for a shared mailbox. Alternatively, the mailbox owner (the primary user) can set the OOA if logged in to the mailbox using Outlook under "Tools => Out of Office Assistant", but you need to know who owns the mailbox, and in some organizations, that is hard to determine.