The FAA has issued an airworthiness directive for certain Bombardier Challenger 300s as a result of reports of deformation found at the neck of the pressure regulator body on the oxygen cylinder and regulator assembly (CRA). It requires an inspection to determine if a specified oxygen CRA is installed and to replace affected oxygen CRAs. Elongation of the pressure regulator neck can result in rupture of the oxygen cylinder and, in the case of cabin depressurization, oxygen not being available when required.