Editor’s note: The following article was written by Christopher Orr.
Update (6/18/11): What follows is an updated version of the original article.
On May 24, 2011 – the feast of the holy Equals-of-the-Apostles, Sts. Cyril and Methodius, Enlighteners of the Slavs and the name day of Patriarch Kirill of Moscow and All-Russia – Metropolitan Jonah (Primate of the Orthodox Church in America) and Metropolitan Hilarion (First Hierarch of the Russian Orthodox Church Outside of Russia) concelebrated the Divine Liturgy at St. Nicholas Russian Orthodox Cathedral (Moscow Patriarchate) in New York City.
This is the first concelebration between the first hierarchs of the Orthodox Church in America (OCA) and the Russian Orthodox Church Outside of Russia (ROCOR) in decades. 
Also concelebrating was Archbishop Justinian of Naro-Fominsk (Administrator of communities in the USA directly under the Moscow Patriarchate), Bishop Tikhon of Philadelphia and Eastern Pennsylvania (OCA) and Bishop Jerome of Manhattan (ROCOR), Igumen (Abbot) Sergius of St. Tikhon’s Monsatery in South Canaan, PA and the former Abbot of the St. Herman of Alaska Monastery in Platina, CA, Archimandrite Gerasim, as well as clergy of the Patriarchal Parishes in the United States, the OCA and ROCOR.
By way of background, the OCA and ROCOR have had a stormy relationship since the latter’s formation in 1921.
The OCA – known previously as the Russian Orthodox Greek Catholic Church in America, or informally as the “Metropolia” – was the Russian Orthodox diocese for North America established well before the Bolshevik Revolution (1917). ROCOR – informally known as “the Synod”, the Russian Orthodox Church Abroad (ROCA), or the “Church Abroad” – saw itself as the duly constituted, representative body of all Russian Orthodox bishops, clergy and laity outside of Soviet Russia based on Patriarch Tikhon of Moscow’s Ukaze (Decree) 362.  The ROCOR hierarchy was primarily comprised of refugee bishops, their clergy and faithful fleeing Russia with the “Whites” who had lost the 1917-21 Civil War in Russia to the Bolshevik “Reds”. However, Metropolitan Platon (Rozhdestvensky) of the Metropolia and Metropolitan Evlogy (Georgievsky) of the Russian Orthodox diocese of Western Europe saw themselves as more ‘canonically established’ than the refugee bishops who had (uncanonically, but understandably) their dioceses in Russia and were without dioceses abroad. That is, Mets. Evlogy and Platon were bishops resident in their own dioceses whereas the ROCOR hierarchs were bishops of dioceses in Russia, which they were unable to occupy.  The Metropolia cooperated with the ROCOR bishops at first but severed relations with them in 1926 citing the Synod’s increasing claims of authority over the more ‘canonically regular’ American diocese. The Synod, for its part, suspended Metropolitan Platon of New York and his clergy for disobedience. However, in 1935, an agreement was signed that normalized relations between the Metropolia and ROCOR, and the Metropolia’s 6th All-American Sobor (1937) affirmed that the Metropolia remained autonomous while reporting to ROCOR in matters of faith.
Towards the end of World War II, ROCOR, which had been cooperative with the anti-Soviet forces of Nazi Germany, was forced to move its base of operations from Yugoslavia (the jurisdiction of the Orthodox Church of Serbia) to New York City (the jurisdiction of the Metropolia).
In November 1946, soon after the close of WWII (in which America was allied with the USSR against Nazi Germany), the 7th All-American Sobor of the Metropolia (comprised of laity, lower clergy and bishops) met in Cleveland and severed ties with ROCOR so as to attempt a reconciliation with the USSR-based Patriarchate of Moscow whose relations with Stalin’s government were greatly improved (comparatively) during and immediately after WWII. Reconciliation between the Metropolia and Moscow was proposed with the stipulation that the Metropolia be allowed to retain its complete autonomy from the Soviet-dominated Church of Russia. When this condition was not met, the Metropolia continued as a self-governing Church in communion with neither Moscow nor ROCOR.
For its part, ROCOR viewed the Moscow Patriarchate as a puppet church controlled by the anti-religious, militantly atheistic Soviet state. ROCOR saw itself as the only free, legitimate part of the Russian Orthodox Church. Some within ROCOR even argued that the Moscow Patriarchate was “without grace”, i.e., no longer Church. ROCOR was constitutionally and culturally opposed to any reconciliation with the Soviet-controlled Moscow Patriarchate.
In 1968, the Metropolia and the Moscow Patriarchate again began informal negotiations meant to resolve their long-standing differences. Representatives from the Metropolia sought the right of sacramental independence and episcopal self-governance (autocephaly), as well as the removal of Russian jurisdiction from all matters concerning the American Church. Official negotiations on the matter began in 1969. On April 10, 1970, Patriarch Alexius I of Moscow and fourteen bishops of Moscow’s Holy Synod signed the official Tomos of Autocephaly, which reestablished communion between the two churches and granted the Metropolia complete autocephaly as the newly renamed Orthodox Church in America (OCA), the fifteenth autocephalous Orthodox Church according to Moscow’s reckoning. ROCOR was decidedly against what it viewed to be the OCA’s compromise with a Patriarchate they saw as being either created or controlled by the anti-religious USSR.
However, after the fall of the Soviet Union in 1991 and the resurgence of free church life in the Russian Church, the canonization of the New Martyrs who suffered under Communism (including Tsar St. Nicholas and his family), repentance over the murder of the royal family, and a general thaw in relations in the first decade of the 21st century, the Russian Orthodox Church – Moscow Patriarchate and the the Russian Orthodox Church Outside of Russia were reconciled in 2007. ROCOR became an autonomous part of the Russian Church.
While intercommunion of OCA and ROCOR laity and clergy has occurred following the 2007 reconciliation , full intercommunion between ROCOR and the Metropolia/OCA in the persons of the presidents of their respective Synods had not taken taken place prior to this historic, 2011 Divine Liturgy. 
“Behold now, what is so good or so joyous as for brethren to dwell together in unity!” (Psalm 132:1)
1. No one seems clear on when ROCOR and OCA/Metropolia bishops last officially (or unofficially) served together in the altar prior to the 2007 reconciliation between Moscow and ROCOR.
2. See the unpublished M.Th. dissertation by Nikolaj L. Kostur, “The Relationship Between the Russian Orthodox Church in North America and the Russian Orthodox Church Abroad from 1920-1950″ (St. Vladimir’s Orthodox Theological Seminary, May 2009), pp. 16-18.
3. As noted in a comment by Fr. Andrew Damick, Met. Platon was also a refugee who had abandoned his Russian diocese (Kherson and Odessa) and found refuge in America where he had previously been diocesan hierarch from 1907 to 1914. After his return to America as a refugee and the departure of Abp. Alexander (Nemolovsky) to Europe, Met. Platon was elected and confirmed as head of the Metropolia by Patriarch St. Tikhon. This appointment was rescinded by later decree of Patriarch St. Tikhon that many took to be written under Soviet duress to Soviet political ends. It became increasingly difficult for Russian hierarchs abroad to communicate with the Patriarchate – and to be sure the communications they received were authentic and freely given. This uncertainty and confusion fomented factionalism and chaos within the Church and emigre community abroad – which was the likely the intent of Soviet ‘meddling’. Met. Evlogy was thus the only hierarch resident in his diocese about which there was absolutely no question regarding his canonical standing, though Met. Platon and the other Russian bishops abroad would dissent the point on various, sometimes conflicting grounds.
The Russian bishops abroad found themselves in a bit of a canonical ‘no man’s land’ since they viewed themselves as refugees who would return home to Russia rather than as permanent residents abroad (or as missionaries). In some ways, with ROCOR being based in Karlovtsy, Serbia, the Russian bishops were hierarchs of the Serbian Church without traditional, geographically-defined dioceses – that is, except for the bishops of the previously established Russian Orthodox dioceses of Western Europe and North America.
This was a confusing time with competing narratives and facts. Time will tell the tale. Thankfully, due to the 1970 reconciliation between the Metropolia and Moscow, the 2007 reconciliation between Moscow and ROCOR, and the 2011 concelebration of ROCOR and the OCA’s first hierarchs the details are now moot outside of academic and historical questions.
4. While not concelebration proper, ROCOR and OCA bishops communed together during the 2010 Episcopal Assembly in New York City. The Liturgy was served by the Dean of Holy Trinity Greek Orthodox Cathedral (GOA) alone with the attending bishops communing in the altar.
5. It has been independently confirmed that individual bishops of ROCOR and the OCA have also served together prior to the May 24, 2011 Divine Liturgy, e.g., the enthronement of the OCA’s Met. Jonah (Paffhausen). It should also be noted that simply praying together – in the altar or anywhere – was an important step for ROCOR and OCA bishops given ROCOR’s stance on prayer with heretics and schismatics. The import of these common prayers was not well noted at the time.