Go to the website of the Orthodox Church in America "oca.org". You'll see the OCA has 3 ethnic (for lack of a better word) dioceses; the Romanian Orthodox Episcopate (now Archdiocese); the Albanian Orthodox Archdiocese; and the Bulgarian Orthodox Diocese. Each of these dioceses have territorial rights throughout the OCA's territory of North America. The Romanian Episcopate was integrated into the Russian Orthodox Greek Catholic Metropolia (the OCA's predecessor) in the early '60's as I recall, regularizing the consecration of Bishop Valarian (D. Trifa) a decade earlier, who was the choice for election to the episcopal seat of the Romanian Episcopate of America, but had been consecrated without the permission of the Church of Romania by bishops of a Ukrainian Orthodox church in the U.S. that was generally not in Communion with the Holy Orthodox Churches at the time. Archbishop Nathaniel of Detroit succeeded Bishop Valarian and continues to lead the episcopate. As the hierarch of highest seniority in the OCA Holy Synod, he has served on several occasions as the Locum Tenens of the OCA's primatial see during the several vacancies since 2008.
The Albanian Archdiocese had been without attachment to the Church of Albania, probably due to the Communist authorities in Albania, but I'm not sure why they were without attachment to a Holy Orthodox Church. Their Archbishop Stephen Lasko was accepted into the OCA in the mid-1970's. His Eminence reposed not long thereafter and the primate of the OCA served as their "Locum Tenens" until Archbishop Nikon of Boston was consecrated as few years ago.
Later, (I think) the Bulgarian Diocese was formed by breaking away from the Bulgarian Orthodox Diocese that was attached to the Church of Bulgaria, again, due to Communism in Bulgaria and its hold on the administration of the church---I'm not familiar with the details of how the break away occurred. That diocese was first affiliated with the Russian Orthodox Church Outside of Russia before withdrawing and submitting to the OCA. Archbishop Kirill of Pittsburgh who reposed a few years ago was their diocesan hierarch throughout their history. Their diocesan offices are in Toledo, Ohio.
While a claim of canonical irregularity of ethnic dioceses overlapping throughout the OCA can be asserted (though the entire organization of Orthodox Churches in North America is canonically anomalous in any event), this structure was the plan (1905?) envisioned by St. Tikhon while he was the ruling bishop of North America, when he proposed to the Holy Synod of the Church of Russia for "some type of autonomy," in order to embrace all the Eastern Orthodox Christians who were emigrating to North America, establishing communities, but were not substantially affiliating with the Russian Orthodox Diocese of the Aleutians and North America, the name of the "Metropolia" at that time.
These ethnic diocese do not participate in the formula employed by the OCA that enables its financial support. Although their hierarchs are fully members of the Holy Synod of the OCA, their dioceses operate essentially distinctly from the OCA.