Hey Vlinder,
I started out with a lengthy explanation of the Churces, but then whn I posted a saw that others beat me to it and actually did a better job. So... I'll shorten this a bit..
The OCA gained it's autocephaly from the MP in the 1970s. This means that the OCA no longer answers to the MP for it is now self-governed having its own head in the Metropolitan (like the Patriarch.. but not so old). The OCA does, however, still respect the MP as her Mother Church.
For a more complete history, there is an excellent online pamphlet of the history of the Orthodox Church in America from 1794-1994 (notice it is missing 10 years... but the info still pretty much applies to today). I REALLY suggest you read this even though it is a bit long. All of it is fascinating. It is divided up into chapters, so I jsut read it in parts, day by day and learned so much!
http://www.oca.org/pages/orth_chri/Orthodox-Church-Introduction/North-America-1794-1994/index.htm (Realy.. read this!!)
One more thing. All of this jurisdictional stuff is very interesting (and sad/pitiful/frusterating at times),
but don't get hung up on it. There was a time when I did. Finally I had to say, look, let the bishops deal with all of this. I personally don't care if someone is ROCOR, MP, OCA, GOA or any other part of the Orthodox Alphabet soup. As long as they are Orthodox, they are my Brothers and Sisters, and everything else is a detail that only adds to the beautiful diversity of our Church (I like Vanshan's Beer analogy lol).
Unity needs to be organic and for that to take place, it needs to start from the bottom up (the layity/clergy to the Bishops) not the other way around. It doesn't matter how many agreements the Bishops sign; if the people don't want unity, it won't happen. Unity is not a piece of paper, but an action in Love and Prayer. If we start this first, the Bishops will ineveitably follow (although it may take a generations or two). This is what has lead the MP and the ROCOR to finally sit down and have serious talks about reunification. This is a delicate issue (with a lot of hurt feelings and distrust on both sides) and they need our prayers so they can humble themselves and embrace eachother once again.
God bless,
servant of God, John