I plan to use an unit of ogres in my next empire army list and have been wondering about fluff justification. Which kind of army would likely include ogres as mercenaries?

Ostlanders tend to use ogres.

Nuln is notable for its employment of mercenaries, as is Marienburg, although Marienburg is not *technically* part of the Empire.

Marienburg not part of the empire?

If you don't want them to actually be ogres, you could treat them as bears and make it a kislev army.

Coming from the Island of Ulthuan, I dont know all to much about the different provinces of the Empire.

Could you tell me which troops (apart from ogres ^_^ ) the provinces you mention (Ostland, Nuln, Marienburg) tend to use most? More cavalry or infantry, what kind of infantry, many or few warmachines etc?

Marienburgers are mercenaries, Nuln are artillery orientated and Ostland are peasant hordes of doom! In other words; both, infantry, infantry.