Unfortunately, bottles are never fitted with temperature gauges, so it's mostly down to guesswork.
Recently, our 744 flight crews have been insisting that pax/crew pressures be at 1800psi or more for certain flights (irrespective of the temperatures) because their manuals tell them it should be this (apparently no temperatures are mentioned). Maintenance manuals/checksheets have the values (with temperatures), so conflict between the two is inevitable. Engineers bow down to flight crew requests, simply because no one really knows what the bottle temperatures are anyway.... especially when you have bottles in multiple locations (at different temperatures).
On a 744, pressure indications can go up 80 psi in a few hours on a warm day (after a long cold flight). Most of the bottles on a 744 are close to the external skin, so get pretty cold.
Pressure/temperature "guesswork" is based on length of flight, location of bottles, ambient ground temperature, length of ground stop, etc.... You'd have to be a rocket scientist to figure out correct values
Also, crew requests at the last minute means the oxygen bottles will be refilled fairly quickly (within safety limits)... This induces a temperature rise in the bottles... and the pressures will drop down after we have finished topping up the bottles.
Unfortunately, it's all very "hit and miss".
Rgds.
NSEU