Both the Oriental and Eastern Orthodox Church worship the same God and the Blessed Mother Virgin Mary and the Saints and the Holy Trinity. Do language and cultural differences separate them? What current issues prevent the churches from being in full communion with each other?
Nestorianism was condemned at the third and fourth ecumenical councils (Ephesus I & Chalcedon I), so calling Oriental Christians 'Orthodox' is incorrect from a mainstream Eastern Orthodox point of view—but this is the title they use for themselves so it is what it is. While some Eastern Orthodox Christians do lump Nestorians and Oriental Orthodox Christians together as monophysite churches, Oriental Orthodox Christians reject this label and distinguish themselves from Nestorians as miaphysite churches.
Concerning Eastern Orthodox and Oriental Orthodox Christians, the difference is that the latter accept only the first three ecumenical councils (Nicaea I, Constantinople I, & Ephesus I), while the former accept seven ecumenical councils. There are some Eastern Orthodox Christians who consider the controversy between Oriental Orthodox and themselves to be primarily semantic (and some dialogue has been made in the 20th century to reunite the churches, but full communion has not yet been restored), but officially they are not considered to be 'Orthodox' (despite their use of the title) as they were condemned as heretical for rejecting the fourth ecumenical council (Chalcedon).
In summary, officially Eastern Orthodoxy teaches that they are the only Church that can rightly be called 'Orthodox,' and thus Nestorians and Oriental Orthodox Christians are actually heterodox. Their refusal to accept these Church councils keeps them separated.