The invention concerns a Venturi device (1) for delivering oxygen or an air/oxygen mixture comprising a main body (10) with an inner compartment (11) and a Venturi nozzle (20); a rear member (30) with an oxygen inlet (31), air inlets (32) and a mixing chamber (33) for mixing oxygen and air. The rear member (30) is mobile with respect to the main body (10) between a closed position wherein the rear member (30) provides oxygen to the Venturi nozzle (20), and an open position wherein the rear member (30) provides an air/oxygen mixture to the Venturi nozzle (20). The rear member (30) is mobile at least in rotation with respect to the main body (10) for allowing a user to manually select a desired open or closed position, preferably according to a helicoidal motion. Installation (100) for providing oxygen or an air/oxygen mixture to a patient comprising a gas reservoir (101), a flexible hose (103), a respiratory interface (104), a connecting hollow body (102), and such a Venturi device (1).