You do share tech from the beginning, which means you'll usually start w/ a scout. If your partner gets 2 of any resource you lack, he'll give it to you the next turn. You also share wonder effects; that is, if your partner builds stonehenge, you get free obelisks. And you share wars; as soon as either partner is at war, both are, and same for peace treaties. (I have not seen my AI partner declare war, actually.)
One minor omission is that you can't control your AI partner's civic choices or religion; you can't even request that he change them. So you'll probably want to choose a teammate whose favorite civics are ones you'll want.
It's quite enlightening to watch how the AI plays. I like to play a OCC game w/ a teammate, against other pairs of AI teammates. (4 teams in small is about right for a fast and fun game.) The AI is awful about taking cities; it basically just creates endless forces and sends them at the enemy. If they are near a city, they usually attack it, regardless of odds. So you end up with lots of defenders getting promotions, which makes success less and less likely. If an AI happens to get lucky it may end up taking a city, but usually it is necessary for you the human to do. In OCC it is particularly hard since you want your AI to capture the city; so you need to knock down the walls, then kill everything inside down to one unit, and hope your bonehead partner wins.