Looking on the internet deeply has found these results:

i am in the office is the most popular phrase on the web.

i am at office 1269,000 results on the web	i am in the office 13,340,000 results on the web More popular!
Some examples and use cases from the internet:	Some examples and use cases from the internet:
Please tell cia... File is locked in steel safe at office With guard night and day. Simplification of transit formalities to be carried out at offices of departure and destination Adam here even took a couple of early runs at Office, But couldn't get elected as a dogcatcher. You said to meet at office hours. Adam here even took a couple of early runs at office, But couldn't get elected as a dogcatcher. At office parties, I rearrange the hors d'oeuvres... At office parties, I rearrange the hors d'oeuvres... while people are eating them so the platters will remain full. I'm no better at office friendships than you are.	It doesn't look like this phrase is very popular! The search could not find any examples on the internet.

i am at office

1269,000 results on the web

i am in the office

13,340,000 results on the web

More popular!

Please tell cia... File is locked in steel safe at office With guard night and day.
Simplification of transit formalities to be carried out at offices of departure and destination
Adam here even took a couple of early runs at Office, But couldn't get elected as a dogcatcher.
You said to meet at office hours.
Adam here even took a couple of early runs at office, But couldn't get elected as a dogcatcher.
At office parties, I rearrange the hors d'oeuvres...
At office parties, I rearrange the hors d'oeuvres... while people are eating them so the platters will remain full.
I'm no better at office friendships than you are.

It doesn't look like this phrase is very popular! The search could not find any examples on the internet.