"At night" is kind of a weird phrase in English (I can't think of any other phrases where at = during). "Om" has 2 meanings in time expressions. (1) om = during: om natten (during the night/at night), om dagen (during the day), om sommeren (during the summer/in summer). (2) om = in: om en uke (in a week i.e. a week from now), om en time (in an hour). "Om" can also mean around (om hjørnet = around the corner) or about (en book om elefanter = a book about elephants). And it can also be a conjunction meaning if/whether.