I am not a teacher.
If you are talking about yourself there is practically no difference between, "I am in the office" and "I am at the office".
If you are talking about someone else, "X is in the office" could be used whether you are in the same building as X or not.
"X is at the office" would be understood to mean that X is at his place of work, which is not where you are.
By the way, "in office" without the "the" has another meaning. A person or group in an official position, often used for the party that forms the present government, is said to be in office.