Learn Finnish: Uutistoimiston mukaan sota on ohi. - According to the news agency, the war is over.

It is required by the postposition mukaan. When mukaan is used to express an information source (meaning "according to"), the source preceding it must always be in the genitive case.

Doesn't mukaan usually mean "along" or "with"?

It frequently does! When used as an adverb (like in tule mukaan - "come along"), it means "with" or "along". However, when functioning as a postposition immediately following a genitive noun (like a person, a study, or an agency), it translates to "according to" that source.

Why does the word for news (uutinen) look like uutis- in uutistoimiston?

This is a standard rule for compound words. Nouns ending in -nen typically change their ending to -s- when they become the first part of a compound word. So, uutinen + toimisto (agency/office) combines into uutistoimisto. The genitive -n is then attached to the very end of the compound.

Is ohi an adjective or a verb form here?

Neither—ohi is actually an adverb meaning "past" or "over". Just like in English, Finnish relies on the verb olla (to be) paired with this adverb to idiomatically state that an event "is over" (on ohi).