An ostomy appliance comprising a front wall and a rear wall of flexible material forming a bag, said rear wall having an opening for receiving a stoma, and said rear wall also being provided with an adhesive wafer for securing the appliance to a user's skin, said wafer having a hole being aligned with the opening for receiving the stoma, wherein the opening has an edge being secured to the surface of the adhesive wafer facing away from the user in an attachment zone surrounding the hole of the wafer, wherein a first part of the edge of the opening of the gab is secured to the surface of the wafer in a corresponding first part of the attachment zone, and wherein a second, remaining part of the edge of the opening of the bag is prepared for adhesive sealing of the same to the remaining second part of the attachment zone renders it possible for a nurse to observe the stoma area during adaptation of such a sealing member when applying one-piece appliances.