I took both, and I don't know if the OAT is a lot easier. It was easier, but I would not say it was a piece of cake. In comparison, I did do better on the OAT, but I had already studied for the MCAT. That makes me think I was just more prepared. As far as length, it is shorter, but there is a reason: 1. no essay section 2. no lunch. That cuts off about 2 hours. Although your MCAT prep will have really helped you, get an OAT book. They are made by different people so the questions are worded differently and you need to get use to it.
Good luck!