Learn Croatian: On je u uredu. - He is in the office.

The base word for office is the masculine noun ured. Since this sentence describes a location using the preposition u (in), the noun must take the locative case. For masculine nouns, this means adding an -u to the end, making it uredu.

How do I know whether to use u or na for a location like this?

You use u (in) here because an office is an enclosed physical space or a room. The preposition na (on/at) is typically used for open spaces, surfaces, or certain public events and institutions.

I learned that u redu means 'okay' or 'all right'. Is that the same word?

They sound similar but come from different words! The phrase u redu (okay) comes from the noun red (order or line), literally meaning 'in order'. The word in this sentence is ured (office). So u uredu means 'in the office'.

Since Croatian often drops words like on (he), can I just say Je u uredu?

No, you cannot start a sentence with the short word je (is). It is a special 'leaning word' (an enclitic) that needs another word in front of it to hold onto for pronunciation. If you want to drop on, you would need to use a different verb structure, so it is best to keep on here.