One of the commenters mentioned hearing from friends about "a lot of extra charges." I did not find that to be the case at all, on my two OAT trips. As with any tour, not everything is included, but most things are, and they are upfront about what isn't:
Meals: The number of meals included as part of the tour varies according to destination. You can find out all this info from the website about the tour that interests you. Anytime you are on a ship (such as the 5 day Nile cruise) all meals are included (and they were excellent). On land, virtually all meals were included in the Egypt tour, while perhaps 2/3 of lunches and dinners were included on the SE Asia one (breakfast was always included at the hotel, and was always good, sometimes downright lavish). All included meals were good, typically the local cuisine at a clean, attractive restaurant (often one with a lot of character), with plenty of food served. Occasionally Western fare. All included meals provided complimentary bottled water, but alcoholic beverages, soft drinks and juices were usually at your own expense, if you wanted them. When meals weren't included, it was always at times and places where there was a good variety of nearby restaurants at various price points.
Tips: You are expected to tip the tour leader. OAT recommends $10-15 per day, per person, given at the end of the tour. The guides were wonderful and worked incredibly hard, and I was quite happy to tip above this recommended range. In Egypt, there was also a tour box for the yacht staff, which I was equally glad to do as they were awesome. Other tips arising out of any of the included services, you are not expected to pay as the tour leader handles that (bus drivers, waiters, local tour guides, hotel porters).
Shopping: This can be a real hassle in some places, such as many locally arranged tours in Egypt where part of the guide's income depends on steering you to particular shops where he gets a kickback on anything you buy (and which the merchant figures into the price he charges you). This was not a problem with OAT on either trip. The tour bus never wasted our time by stopping at any alabaster workshops, "papyrus museums", perfume factories, or anything like that (of course, we went plenty of places where there were things to buy, but not places that appeared to be solely aimed at getting us to shop). The guide was available to give good advice to those on the tour who did want to shop, but there was no pressure whatsoever on anyone to do any shopping or to purchase anything.
Excursions with surcharge: Virtually everything the group does and sees is included in the price (of course, if you want to do other things on your free time, those are at your expense, but most of each day does comprise included activities). There are a couple of additional ones offered if you want, but few. On the Egypt trip they offered a balloon flight in Luxor, and in SE Asia, one trip to an outlying temple near Angkor Wat in addition to the main ones included in the tour, and an evening food tour by motor scooter (local drove the scooter, tourist on the back) in Saigon. I did these, except for the food tour, and thought the surcharges were reasonable.
Also I liked the group "vibe" on both trips. Good people. I'm travelling with OAT again soon, to the Balkans, and looking forward to it.