The Ecumenical Patriarchate of Constantinople

Symbol of the Ecumenical Patriarchate of Constantinople, as found atop the front entrance of the Patriarchal Cathedral of St. George.

OIK has the meaning of “house of God”

Double-headed eagle:
This is traditionally a symbol of the Byzantine Empire and later adopted by the Eastern Orthodox Church and the Russian Empire. It often signifies imperial power and the unity of church and state (one head facing east, the other west).