Abstract: An oral-nasal device includes a face mask and mask-securing means. The mask has a first portion formed with a cavity to accommodate the nose of the wearer and overlie his mouth, a second portion that is a compliant seal surrounding the cavity, and a shoulder surrounding the cavity and overlying the seal. The mask-securing means includes a pressure plate which in use forces the seal to conform to the facial features of the wearer and to be continuously sealed thereto. In the disclosed embodiment of the invention, the mask-securing means is secured to a diver's helmet.