A bit of an expansion:
The OCA ultimately originated in the Russian Mission to North America. When the Tsarist government fell and the communists came to power the Patriarch of Moscow declared that those churches outside of Russia should break off communication (note: Not Communion) with Russia until such a time as it was safe for them to resume it.
In response to this move almost all Russian churches outside of Russia did so, leading to two autonomous churches, one was simply known as the Metropolis of North America, and the Russian Orthodox Church Outside of Russia. It is also at this point that other Orthodox Churches began to be established in North America.
Later the Metropolis would be augmented by a group of break-away Eastern Catholics and would rename itself the Russian Orthodox Greek Catholic Church.
In late '60s, tensions were cooling in the Cold War and negotiations between the ROGCC began. By 1970 the two groups announced a Tomos of Autocephaly. This document granted the ROGCC autocephaly under the name “Orthodox Church in America”, and handed over the bulk of Russian diocese over to the OCA (I live in one of the areas that was exempted from this, so you can find Churches under the Moscow Patriarchy in the little towns around here).
One issue that often causes confusion is that a number of Orthodox Churches don’t recognize the autocephaly of the OCA. They all recognize it as Orthodox, they just disagree on whether it falls under the description of Autocephalous, or Autonomous. This is not a distinction that should really matter to anyone outside of Orthodoxy. It is Orthodox.