The Orthodox Church is unified, unless you are thinking about the Eastern Orthodox and the Oriental Orthodox. But they are really two different Communion of Churches, much like the Roman Catholic Church and the Old Catholic Church. Though they are both Apostolic Churches and once were in communion with one another, they have been in schism from one another for 1500 years. Though surprisingly the faith is pretty similar. The disagreement lies in Chalcedon, regarding the 2 natures of Christ. The Eastern Orthodox (and Roman Catholics) accept Chalcedon that says Christ has two natures, human and divine. The Oriental Orthodox has a similar belief yet laid out differently, that Christ has one nature though that one nature is both completely human and completely divine. The schism that ensued also caused a lot of bad blood between the Churches and they have been trying to sort those things out, because the Fathers of one side have condemned Fathers from the other, so even today when they have pretty much patched up their theological differences, there is still a lot to sort out before full communion is restored.
