I also have Oxford/UHC and my plan docs say, "Documentation of a motivated attempt of weight loss through a structured diet program, prior to bariatric surgery, which includes physician or other health care provider notes and/or diet or weight loss logs from a structured weight loss program for a minimum of 6 months."
I am going to see my nutritionist rather than do WW partly because my ins covers nut visits but also less paperwork for me to gather!