I'm interested in Old Testament (or Judaic) rules and customs preserved in different ancient Churches. As far as I can understand, they are (or were in the past) more preserved in OO Churches, maybe because of being centred mainly in the Middle East.
I'll mention some that I myself know and am waiting for others to add more examples.
I kindly request the members of this community to not make this thread another place for dogmatic debates.
As far as I know (if I'm wrong, correct me, please), the Ethiopian Church maintains the law of circumcision and some others too, but I don't know which ones.
In the Coptic Church, deacons are not allowed to incense, and only priests are allowed to do this because it's prohibited in the Old Testament for non-priests to incense. If not all the Copts, at least their monks and nuns shun mixing different fabrics according to the Old Testament rule about this.
In my mother Church, the Armenian Church, the dietary rules of the Old Testament were preserved but since I don't know when they have simply fallen into disuse. And most of Armenians even don't know that officially or traditionally their Church follows the Old Testament food rules and it's prohibited to eat the unclean animals, meat with blood or of strangled animals. According to the Armenian Church rules, also a food into which mice or other such animals have fallen is considered defiled and uneatable.
The consecration of church buildings and vessels, icons, and also the ordination of priests and other ecclesiastical orders, including the patriarch, are done using the holy oil, as it was in the Old Testament. Moreover, in case of priests the oil is put in their palms, while in case of the patriarch it is poured on his head, just like what was done to Aaron and his sons, and what is prescribed in the Old Testament about this.
The Armenians also mourn their reposed 7 days, instead of 9 days of the Eastern Orthodox. Well, this is not an Old Testament regulation, but is a Judaic custom.
Now, I'd like you to continue with the examples. Especially I'm interested in Old Testament customs of the other Oriental sister Churches and, why not, also, of the Assyrian Church of the East, if there are people here from it.