There is provided a disposable ostomy bag assembly (1) comprising a bag (3) for receiving bodily waste materials; a compactable sheath (2) for closing and sealing the bag (3) after use; and a flange (4) for securing the assembly to the body of a patent and provided with an orifice to enable bodily waste to be received by the bag (3). The ostomy assembly (1) solves the problem of escaping malodor and bodily waste when the patient detaches the ostomy bag from the body (face plate (5)). The compactable sheath (2) can be stretched to acquire a cylindrical sheath (2) extending from the flange (4), whereby the sheath (2) may be closed in order to seal the contents of the bag (3). Closing the sheath (2) may simply be done by tying a knot (6).