Did a safari about 5 yrs ago, not with OAT. I was careful to select a company with small vehicles, only 4 of us in the land rover. I saw other rovers with 6 and even small tour busses with over a dozen. You need to be very clear with your tour company about the type of vehicle as you are going to spend hours in it. You want a max of four people (plus the driver/guide) with an open top. Some people prefer a top that extends upward which provides shade for those hours you are out in the sun. Others prefer a removable top for its better visibility and you wear a hat.
You do NOT want to be in a big bus or crammed into a vehicle fighting for views. Look at the lodging options. There are tents and there are tents. Most tour companies can arrange a choice of basic to luxury. Are you interested in spending a whole day of your safari visiting a local tourist trap "craft" factory? or a school? I looked at the OAT itinerary and it looks like of the advertised 13 days there are only two full days and 3 1/2 days of wildlife time. I would encourage you to look at two other tour companies, Roy
Tanzania Safaris, Safaris in Tanzania - Roy Safaris, Arusha, Tanzania Safari Specialist and Good Earth
http://www.goodearthtours.com/ Both of them will create a trip exactly as you wish. Good Earth has a Florida based representative although it is a Tanzanian company. Check Fodors and Tripadvisor websites for reviews. Of course very few people do this more than once so they can't really compare one experience to another. Everyone seems to come back amazed at how great it was no matter what company they use. The animals are the same for everyone so look at the vehicles, time spent sitting in hotels and driving vs out there seeing, and decide on safari time vs other activities. Go to the tripadvisor website and review different tent camps and lodges and look at the pictures to see what is available. However you do it, it will be a great trip. If you wish to PM me I can give some additional guidance.