I was just curious when the Eastern church started to refer to itself as Orthodox and how the Non-Chaledonians referred to themselves as Oriental Orthodox and when the Western church referred to itself as Roman Catholic?
From what I can understand, the term "Orthodox" was used from the earliest times throughout the Christian world by those Christians who wished to distinguish themselves from various heretical groups, or just as a means of identification. It was only later in the West (though I don't know at what point exactly) that the term "Catholic" came to be used instead. IIRC, the Oriental Orthodox use this designation just as a kind of convenience, just like the Eastern Orthodox do, in order to make identification easier for outsiders. Both the OO and the EO consider themselves to be simply "Orthodox". Of course, the Roman Church also considers herself to be the Orthodox Church, but stresses its belief in its catholic component instead. (And, I might add, its own particular understanding of the term, ie "universal.")
"Byzantine" Greeks actually considered themselves to be Romans, of course. Orthodox people in various parts of the Mediterranean basin still refer to themselves as "Roman Orthodox" today.