Oriental orthodox is not a religion, it is a catch all name for multiple groups. Armenian, Coptic, Assyrian... are as different from each other as they are from Catholicism
As far as I know, only the Assyrian churches are actually distinct (and thus not included inside this label). The Armenian, Coptic, Ethiopian, Eritrean, Syriac and Malankara churches are in full communion with each other, and that's why they are being grouped as Oriental Orthodox.