D.I.S.S. stands for a Diameter Index Safety System - a type of outlet valve that provides a unique connection that is specific for the gas used (i.e. oxygen, CO2, nitrogen, etc) to avoid delivery of incorrect gas. A D.I.S.S. valve is compatible with regulators who need to be attached to a humidifier bottle for added moisture for the oxygen patient. A D.I.S.S. must attach to either a humidifier bottle or a swivel oxygen connector (Christmas tree) in order to be used with an oxygen cannula. You cannot hook a cannula to a regulator with a D.I.S.S. valve without one of these two adaptors added to the regulator. Most of the regulators with a D.I.S.S. valve are used on the larger tanks (larger than E that are primarily used for back up in patient homes) with a CGA 540 connection.
A barb connection is standard for most of the regulators used on portable oxygen tanks and can connect to oxygen cannulas with ease, no additional adapters needed.