No idea about the St Regis, but we split a week between the Oberoi and O&O 2 years ago.

The O&O was very good and had that resort feel to it. Lots of people, queues for the buffet at dinner (though there were non-buffet options if you prefer), entertainment (music and traditional dances) in the evenings. The beach was amazing and we enjoyed it very much.

The Oberoi on the other hand was much quieter. We'd often have one of the swimming pools completely to ourselves (despite the restaurant being full at dinner, so there were plenty of other guests). We'd also end up being the only people left in the bar by about 10pm. We loved the food there and, again, enjoyed it very much.

Overall, we found the Oberoi won in most respects other than the beach (which is ok, but nothing in comparison to the O&O) but the hotels are very different to each other.