We've taken a combined 9 trips with Grand Circle and OAT, and some time ago, we noticed that Grand Circle and OAT sometimes have trips which cover many of the same venues. We decided to look into the difference, and what we have found may be useful to travelers deciding between the two, when the intineraries appear to be quite similar. Grand Circle trips are generally less strenuous and do not have any 'adventurous' esxperiences, such as hiking or any kind of physical activity beyond getting on and off the bus. OAT, OTOH, may involve some hiking, walking over unpaved and back country roads and trails. Additionally, OAT clients may visit more 'rustic' areas and get more involved with local people than will clients of Grand Circle. Grand Circle clients will visit more upscale restaurants while OAT clients will visit more authentic restaurants. Regardless, the food is good on either trip. Ditto for the hotels, although all hotels are up to and excede western standards, regardless of whether you choose Grand Circle or OAT. Additionally, OAT groups are smaller( never more than 16 ), while Grand Circle trips have up to 40. You'll get closer to your program director on an OAT trip, although you won't be ignored as an individual on a Grand Circle trip; the program director will still send you a welcome e-mail and will know your name by the time you get on the bus for your first experience.
Of course, OAT will be a bit more epensive than Grand Circle primarily due to differences in group size, and your tips will be higher than with Grand Circle, pretty much for the same reason. But the fundamental difference is in the nature of the experience you will get. OAT offers are more authentic experience, while the Grand Circle trips offer you more of an experience as a 'tourist.'
So, when planning a trip and deciding between the two, if both offer a trip to the same locale, choose according to the type of experience you want in that part of the world. Regardless of which you select, you won't be disappointed. We never have been, which is why we have take 9 trips, with a 10th upcoming, and continue to travel with Grand Circle/OAT( we have pre-booked an OAT trip to Nepal for 2013 ). We cannot recommend them more highly.