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 the office 111,700 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:
As and damage and money Mrs. Patrick McNeilly and I am at the office of good clinical practice. This will of my pleasure to be here to discuss this topic. Jan 27, 2015 ... I'm thinking like 3 max on my right hand and I would not wear them to court, just on days that I am at the office. What do you think? We've talked ... Apr 7, 2011 ... I listen to music when I am at the office late at night or on weekends, as I need a little white noise / distraction or I get crazy. Between 8am – 6pm ... Dec 14, 2006 ... I am "at" the office now. I am "in" the office now. It means the same thing, and I suppose it is a personal preference on how it is said. lfeat.	It doesn't look like this phrase is very popular! The search could not find any examples on the internet.

i am at the office

111,700 results on the web

i am in the office

13,340,000 results on the web

More popular!

As and damage and money Mrs. Patrick McNeilly and I am at the office of good clinical practice. This will of my pleasure to be here to discuss this topic.
Jan 27, 2015 ... I'm thinking like 3 max on my right hand and I would not wear them to court, just on days that I am at the office. What do you think? We've talked ...
Apr 7, 2011 ... I listen to music when I am at the office late at night or on weekends, as I need a little white noise / distraction or I get crazy. Between 8am – 6pm ...
Dec 14, 2006 ... I am "at" the office now. I am "in" the office now. It means the same thing, and I suppose it is a personal preference on how it is said. lfeat.

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