All relevant product information can be found on the Analox Sensor Technology website, take a look at the O2NE+  and Safe-Ox+ product pages.  

The O2NE+ devices use partial pressure oxygen sensors and  integral pressure sensors that enable them to automatically compensate for changes to ambient pressure. This means the O2NE+ and Safe-Ox+ units can be used at atmospheric pressures anywhere between 811 to 1050 mbar absolute!