Yes, I do have the 2nd conformation that is for the OS segment. When I go to their site and lookup the booking, I get the message "At the moment we are not able to offer online seat reservations for your booked flights."
(Also, on the OS site, it says hand luggage 5kg only.. should I be concerned?? On united.com, it shows that segment as 1 bag up to 50kg free)