in office—
1. inside a room used as a place of business:
- When the boss is in office she does not take lunch, comes in extra early and leaves late.
2. in an official position of duty or authority:
- Each President will be in office for five years, and one person can be elected as President only twice.
in the office — = in office 1:
- Just because the boss is not in the office doesn’t necessarily mean he is not working.