I'd also be interested in your experience with OAT- I also took them in India and Nepal just this past March and loved them. The primary guide was great about answering all questions- even some of the good/bad/ugly ones about the country- tactfully and honestly. He was also very responsive to different things people wanted to do- a special excursion, stopping at the ATM, sitting out a rigorous hike- always making sure they had a god place to stay and a way back to the hotel (which he'd arrange and prepay). I asked if there was a way I could deposit some of my late husband's ashes in the Ganges. We had a ride on the Ganges scheduled at Varanasi anyway but he brought a Hindu priest with us who chanted from the Vedas as I released a dish made of pressed leaves, filled with a few flowers and some of DH's ashes into the Ganges. I'm a Christian and it was PROFOUND- one of the most memorable experiences in the trip. No single supplement- a huge benefit to me.
I'm not a "group" traveler- happy to explore Europe on my own and have done so, many times- but I'd been to India on business often enough to know it wasn't a good place to solo. Too crazy to drive yourself, the trains aren't great (the one we used on the tour had tray-tables that hadn't been cleaned in awhile and was equipped with squatty-potties) and just hiring a driver to take you around is isolating. The guide also knew which gas stations had Western-style "facilities" and would tip the appropriate people and wait till we got out. I don't need that kind of babysitting in Europe or South America but definitely appreciated it in India- especially when I developed a bad case of dehydration (despite my best efforts to prevent it) and he came up with envelopes of electrolyte powder. Magic stuff.
Will probably use them for the Galapagos in 2020.