Looking on the internet deeply has found these results:

I used to work in an office is the most popular phrase on the web.

I used to work at an office 1227,000 results on the web	I used to work in an office 1578,000 results on the web More popular!
Some examples and use cases from the internet:	Some examples and use cases from the internet:
It doesn't look like this phrase is very popular! The search could not find any examples on the internet.	And here's a guy that used to work at an office, and he's really embracing masculinity. To tell the truth, I used to work in an office. Well, I used to work in a factory that made those. Don't worry... I used to work in the infirmary. I used to work in the corporate world... as an office manager. I used to work in this warehouse when I was in college. I used to work in the VP's office during home room. Who used to work in an office but is now a traveler.

I used to work at an office

1227,000 results on the web

I used to work in an office

1578,000 results on the web

More popular!

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

And here's a guy that used to work at an office, and he's really embracing masculinity.
To tell the truth, I used to work in an office.
Well, I used to work in a factory that made those.
Don't worry... I used to work in the infirmary.
I used to work in the corporate world... as an office manager.
I used to work in this warehouse when I was in college.
I used to work in the VP's office during home room.
Who used to work in an office but is now a traveler.