"What Is a Colostomy? A colostomy is an opening in the belly (abdominal wall) that’s made during surgery. The end of the colon (large intestine) is brought through this opening to form a stoma. Where the stoma will be on the abdomen depends on which part of the colon is used to make it. Some colostomies are large, some small; some are on the left side of the abdomen, some are on the right, others may be in the middle. . When you look at a stoma, you are actually looking at the lining (the mucosa