Until a dental practice has at least a half dozen staff or more there is no real need for designating any employee as the office manager as what a real OM does is hire, train, apprentice, correct and manage the other staff.
A real OM is not answering the phone, scheduling, doing financial arrangements, etc. However an OM should be able to do all these jobs better than anyone else otherwise how can she properly train, correct and manage the other staff? A vital skill for of an OM is having the expertise to train other staff.
The truth is when you have willing staff members that have been trained on proven systems then they require almost no supervision or management. In essence they become executives of their own areas. When this occurs you will have a staff driven practice instead of a doctor driven practice, which is a beautiful thing.