The Eastern Orthodox and Oriental Orthodox have been two separate churches ever since the Oriental Orthodox refused to sign onto the Council of Chalcedon in the 450's AD, because they felt that its rulings and canons had some Nestorian tendencies, which isn't the case. On the same token, the Eastern Orthodox accused the OO of being Monophysites, which they actually aren't. Basically, the only thing keeping our two churches apart are mere semantical differences in our Christology (hierarchs from both churches have agreed that we share the same Faith), historical awkwardness, some bad blood and feuding, and deciding how the hierarchical situation would be resolved (i.e. both the EO and OO have a Patriarch of Alexandria, and we're only supposed to have one bishop per city)