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

Both the O2NE+ and Safe-Ox+ units require a warm up period of approximately 2 hours. They are expected to read slightly higher at power on than they will once they have stabilised.

When calibrating the units, it is best practice is to power the devices and leave them to warm up for a minimum of 2 hours, before carrying out the full calibration. 

This is also true for normal use, power the devices up and allow to warm for a minimum of 2 hours prior to considering the readings stable.