"om" is used in the following cases:

• om morgenen/formiddagen/eftermiddagen/aftenen/natten (Times of the day. Only for time spans, i.e. only the five mentioned.)

• om mandagen/tirsdagen/onsdagen/torsdagen/fredagen/lørdagen/søndagen (Days of the week)

• om sommeren/efteråret/vinteren/foråret (The four seasons)

I think "i" would mean this afternoon, specifically.