From my limited experience of Oriental Orthodoxy (attending several Coptic liturgies and one Indian Orthodox liturgy, as well as watching Youtube videos) it seems that the practice and worship vary much more between the OO Churches than between the EO Churches. While there are Greek and Slavic forms of the Divine Liturgy, it's still the same Liturgy and all EO churches that I've seen pictures of have the basic layout (iconostasis and square altar). However, the Coptic Church uses the Liturgy of St Basil and the Indian Orthodox Church uses the Liturgy of St James. The Ethiopian and Armenian liturgies seem to be different again. The Coptic church I visited has a similar layout to EO Churches (although no royal doors, just a curtain) with a square altar and iconostasis, but the Indian Orthodox church had an altar very similar to a pre-Vatican II Roman Catholic altar but with no Tabernacle and more candles. It was simply closed off by a curtain.
What is the reason for the disunity among OO churches on things like Liturgy and church layout? Why aren't these things more unified like in the EO Church?
The reason we all in the EO celebrate the Divine Liturgy of St. John Chrysostom in the manner we do is because of the Byzantine Synthesis. It was the movement to make the rite of the "Great Church" (Hagia Sophia) to be the rite of the Eastern Church. This movement didn't occur until after the schism with the Oriental Orthodox. The other reason is that the Slavic peoples were under the jurisdiction of the Patriarch of Constantinople, whose church celebrated the Divine Liturgy of St. John Chrysostom.
You also have to understand that the title of "Greek Orthodox Church of Antioch", "Greek Orthodox Church of Alexandria" and "Greek Orthodox Church of Jerusalem" really are accurate in many aspects. The chant, and even to a degree, the ethnicity of the members of those churches is Greek, and they were very heavily influenced by the Constantinople Patriarchate.
So really, what you see as "unity" is really just a conformity of liturgical rite which arose from various historical circumstances. In the Eastern Orthodox Church, instead of going the route of the Roman Church, which sought both liturgical uniformity and linguistic uniformity, the Eastern Church instead sought liturgical uniformity while promoting the vernacular language and permitting local traditions.
However, in the ancient Church, and until the Byzantine Synthesis (and in the West, the Roman Rite Reforms) there were dozens, if not hundreds of various rites. What we call the Divine Liturgy of St. John Chrysostom was simply the rite which was practiced in Hagia Sophia. This was essentially derived from the Liturgy of St. Basil and the ancient liturgies of the Holy Land. However, it was, in its own way, somewhat unique from the other churches.Disclaimer, I may have read wrong about this, and so don't take the following as complete fact:
For example, the Great Entrance is somewhat unique, because in many other Christian Churches at the time, the gifts may have been stored in the Church itself. However, Hagia Sophia had a "skeuophylakion" which stored many of the treasures of the Patriarchate, and I think this included the chalice and plate for communion. The Great Entrance was literally an entrance where the gifts were brought into the cathedral from the skeuophylakion. Many other churches probably didn't have this great entrance because they didn't have a skeuophylakion as a separate building. Today, the gifts are kept in the altar in the prothesis (which is sometimes a separate apse next to the altar) but the Great Entrance is kept.