A question for Orthodox Christians?

Wether Russian, Greek, Antiochian, OCA or something else, it's all Eastern Orthodox. If the priest is Eastern Orthodox, he can serve at any Eastern Orthodox Church, no matter the ethnicity or where it came from. It's Eastern Orthodox just the same. The only difference between the different churches are the style of singing and some have unique traditions (Such as what you include in the traditional Paschal Basket and what they represent). If he was ordained in a Greek Orthodox Archdiocese, the priest is Greek Orthodox, but he can still serve anywhere Eastern Orthodox. In other words, yes, he can be a priest in the Russian Orthodox Church.