Do modern airliners have oxygen sensors in the cabin?
While it would not significantly affect the O₂ percentage one thing that must be tabulated on a cargo aircraft is the amount of dry ice loaded. Dry ice is very common in packaging to keep perishable shipments cool during transport. Since dry ice sublimates into gaseous CO₂ as it warms there is a limit to how much can be loaded into an aircraft. If there is too much there's a possibility that the pilots could be overcome by gas trapped inside the plane. It's a rather insidious threat because you can't smell it so you would only realize something was wrong when you started feeling physiological effects. 

An O₂ sensor would not help since pilots would be overcome long before it made any detectable difference in O₂ levels. AFAIK there are no sensors for CO₂ either. The amount of dry ice loaded is carefully tabulated and pilots who fly cargo containing dry ice are aware of the possible hazard and how to recognize symptoms of CO₂ poisoning.