At work place, usually the calendars are shared publicly by default. So that we can coordinate with people accordingly. But sometimes you sync your professional and personal appointment to a single calendar to manage them easily and to look up for a full schedule in a single calendar. So in order to hide your personal appointments from the shared calendar, set them as "Private".
If you add a new appointment to Outlook calendar, then there is no direct way of marking it as private. But as Outlook can be customized, so you can make changes to default appointment form in order to customize it, in your own way.
For Outlook 2010
For Outlook 2007
For Outlook 2003 and previous
For Outlook 2010 and 2007
Now when a New appointment is opened, there will be a by default private button enabled on it. And to disable it, click the button once again.
This method is not supported by all versions of Outlook and Exchange versions. There a choice of calendar permission level is given. Either you can set it to "free or busy time, subject, location".
Another option is to separate calendar as Outlook and mobile calendar application can access or subscribe to multiple calendars and viewed them in a combined mode, it would appear as single calendar which ensures you that office members don't have any access to it.