While trying to book today a CC flight, I noticed the difference between the O and U fare:
- O books in comfort class for long-haul flight and in economy for the short-haul connecting flight
- U books in comfort class for long-haul flight and in business (J) for the short-haul connecting flight.
In my case 1hr to IST is not relevant, but if you get new J short-haul seat-plan, this is a nice option to take.