The Opus 15 and Dweller 15 (13's also) are the same unit, just refreshed. Opus must have picked up the rights to that.
Read through the OBI thread, it will give you a lot of insight, obviously some things have changed, but over all it should give you an idea of the units.
I've spent time in the Opus 13, one the local dealers has one. Very nice, lots up lay out tweaks that make it better in a lot of ways than the OBI unit. That said the OBI unit has some nicer items.
13 only items changes:
Interior stove and sink, just takes up space vs storage (13 only).
Storage change on the passenger side, more exterior storage, but takes away interior storage (13 only).
Better:
Cabs are a nicer wood.
AC is roof top
Kitchen lay out, and space next to kitchen storage (exterior).
Bar deck is much better.
Worse:
Regular rv door vs the OBI door.
Black tank vs cassette (this is a major thing IMHO, but the us market is used to black tanks).
Front storage/rock guards not nearly as nice as the big bin on the OBI.
For the 15' and maybe the 13' both; the dual rear tires and jerry can. Dual tires is heavy off the rear.
Over all Opus makes great rigs. I'm glad the 13 and 15 designs found a home with them, they are great trailers.