Rub brisket with barbecue rub and place in oven cooking bag.
In small bowl, combine onion soup mix, onion, barbecue sauce, 1 cup water, Worcestershire sauce, and soy sauce. Add mixture to oven bag with brisket. Seal, and refrigerate overnight or at least 8 hours.
Preheat oven to 250 degrees.
Place brisket in oven bag, fat side up, in a roasting pan. Bake until tender, 8-10 hours.
Remove brisket from sauce, reserving sauce. Slice brisket across grain. Strain sauce, reserving liquid and discarding solids. Skim fat from sauce and serve with meat.