We always say "på kontoret" but "i kontoret" is not grammatically wrong and you would be understood.
oh, thank you! is there are reason or pattern for preferring "på" though? i'm not sure which situations which technically should be used in
Even as a native i have never been able to see any logic or pattern in it, i have simply gotten used to it. But here is a list with many examples for you to study.
Very useful. Mange tak. Note, however, that -- strictly speaking -- this table is about words for "to" (to work, to school, etc.) rather than "at/in" (at church, in England, etc.).