Language/English/Vocabulary/“In-the-office”-or-“at-the-office”

“In the office” or “at the office” (note the definite article!)

“I am in the office” means that the office is a room and I am inside that room. However, the word “at” expresses the general idea of the location of one’s office and is often interchangeable with “at work”.

Compare:

• I am in my/the office. = My office is a room and I am in that room.

• I am at my/the office. = I am somewhere near my office or in it. I am at work. 

"at my/the office" is much more common in British English whereas "in my/the office" is more used in American English.

There's a reason for using "the" for this case. "I am at the office." is really short for "I am at the office where I work.". In the context of conversations about office spaces and working, "where I work" is really redundant. So, we just drop the "where I work" in normal conversation and end up with "I'm at the office.".

“In office” (without an article) implies something relatively different. When you say that somebody is “in office”, it means that they work in an official position, usually for the state as authorities.

Example:

• He, as a president, was in office from 2000 to 2020. 

Finally, “at office” (without an article) is not commonly used. However, If you want to say “at office”, it is better to say “at the office” instead.

Example:

• I am not at the office right now. (not I am not at office right now).  

More information on how to use ON, AT or IN : https://polyglotclub.com/wiki/Language/English/Vocabulary/How-to-use-ON%2C-AT-and-IN