Now you have let the cat out of the bag Tightslot. We will have to resort to chloroform from now on!
Without delving too much into what has already been said about the composition of air, recycling and the partial pressure reduction effects, in fact when an aircraft takes off and power is applied to the engines, the cabin pressure actually increases above ambient pressure. It does this to ensure a smooth transistion in the pressurization process and also to help ensure the door seals are more effective than they might otherwise be, during the take off and early climb.
So albeit rather academic, you could actually say that the reverse was true in that for a given volume, there are actually slightly more oxygen (and other gas) molecules during take off, inside the cabin than there are outside.