It's kind of in a tough spot, officially, my pants should ride on my hips right where the ostomy is, but since it is in the same place as yours, I let them sag most of the time.
as for underwear, I wear boxer briefs, most days, and i leave the end with the clip or closure outside my underwear, between the underwear and the pants. You always have to make sure you have an under shirt tucked in, or a long enough shirt on top, or people will see your secret when you stretch or life your arms.
I am fairly athletic, but most of the time, my bag setup the way i described above is just fine, only thing is, when I am doing stuff like that, I drain often so I don't run the risk of extra pressure on the seal, and also since it is easier to hide an empty or barely full pouch.
Good Luck and come again....
"I am he as you are he as you are me and we are all together!" - The Beatles
doesn't that just sound cool when you sing it?