Ogre camps have a chance to spawn whenever an act of destruction takes place – battles, sieges, the razing and sacking of settlements, anything where there might be plunder to be had and meat to be served. Ogres will move into an area and create a camp, which you can then interact with via an army. Here you can choose which tribe to work with and pay, giving units to the local mercenary recruitment pool. From there, you simply have to actually recruit the units into the army you wish to have them in, and then keep them paid. The Ogre camp will disperse, though more can always spawn later. If you run out of money to pay your Ogres – who are not cheap – then they will depart without a moment’s hesitation.