I am a member of the Greek Archdiocese and I am wondering why musical instrument such as organs are not found in OCA Churches, or in Churches I visited in Greece or on Mt Athos? It seems Psalm 150 condones praising the Lord with stringed instruments.
Actually, the tradition of the Orthodox Church is to have no musical instruments in the church. This is not unique to the OCA, as you experienced during your trip to Greece and Mt. Athos. The appearance of organs in Orthodox churches is, to my knowledge, pretty much limited to Greek and a few Antiochian Orthodox parishes in the US. I have never heard of organs or other musical instruments used elsewhere. Hence, the use of organs in some churches in the US is an innovation of recent origin. If I am not mistaken, His Eminence Archbishop Spyridon of the Greek Orthodox Archdiocese of America has addressed his concerns over the introduction of organs into the church’s worship.
It is true that Psalm 150 condones praising the Lord with stringed instruments. This, however, never carried into Christian worship. One could say that there are many things one finds condoned in the Old Testament—the sacrificing of animals, for example—which were not carried over into the New Testament Church, in which Christ becomes the sacrificial lamb and the human voice becomes the musical instrument par excellance.