The ornamental design for an ostomy bag, as described and shown. The entrance for a stoma and the adhesive member on the rear surface of the ostomy bag do not form part of the design.
FIG. 1 is a front view of an ostomy bag;
FIG. 2 is perspective view from the front and the bottom of the ostomy bag of FIG. 1; and,
FIG. 3 is a rear elevation view of the ostomy bag of FIG. 1.