Ogres live in Bal Rattha the desert part of the map. If you send an army there with units strong enough to take down the Ogres you can tame them by attacking them, it will be a random choice if they join you or not. They make excellent chock troops and carriers.
Simply go into "View" with your army in Bal Rattha and start attacking Ogres, if you want you can set up a camp there too. If you are Elves or have hired Female wardens with humans or orcs they can convert the food on the dead ogres to gold boosting your gold income, if you want the food from the Ogres simply use any other worker unit.