I am confused by the focus on Judaism in this thread.
Anyway, as a non-Catholic, who must I obey…it’s an interesting question, with a few possible answers. While in Orthodoxy (by which I mean Orthodox Christianity, to clarify in light of all the Judaism talk that is going on in here) there is not the centralized, top-down model of authority and organization that is found in Catholicism,
I still feel like it would be appropriate to say “the Pope”, in light of the fact that the Coptic Orthodox have a Pope, too.

His role is not exactly conceived of as the Catholics might conceive of their Pope’s, but anyway…certainly what HH Pope Shenouda III says is not to be discarded or treated trivially. As the bishop of Alexandria, a stalwart defender of the Orthodox faith, there can be no doubt as to the weight and wisdom of theological insights, opinions, and rulings. That said, I don’t doubt either that the ultimate allegiance is
to the faith itself as passed on by his predecessors, and their predecessors, going back to the days of St. Mark the Evangelist and the other apostles and disciples of Christ. I can be confident without knowing anyone’s mind that this is the answer that would be given by HH himself, if maybe not in those exact words, since his words would be in Arabic.
Anyway, the ultimate point is as always in the command of the Savior: “If you love Me, you will keep My commandments”. So from a certain view the answer is a “who”, but more broadly it is a “what”, too. And the commandments as distilled by the monks into bare essentials of what is necessary for salvation have on occasion been so simple as to almost seem like a trick, but are really enough to answer the OP’s question in two sentences. spoken some ~1700 years ago by St. Anthony the Great, the Father of Christian monasticism: “This is the great work of a man: always to take blame for his own sins before God and to expect temptation to his last breath”, and** “Do not trust in your own righteousness, **do not worry about the past, but control your tongue and your stomach.”
If there’s a better way to put it, I haven’t heard it.