Do pilots get more fresh air than passengers?
Taking the B737 as an example, fresh air from the left pack flows directly into the cockpit (green airflow in the diagram), whereas the passenger cabin is served from the mix manifold only (yellow). Therefore


pilots receive 100% fresh air
while passengers receive a mix of 50% fresh air and 50% re-circulated cabin air on average, across all aircraft types



share of re-circulated air (red) in the cabin ventilation air (yellow):

B737: pilots 0% passengers 25%  
B757: pilots 0% passengers 50%  
B767: pilots 0% passengers 50%  
B777: pilots 0% passengers  ?


In a cabin packed with hundreds of people each transforming 21% O2 and 0.04% CO2 into 16% O2 and 4% CO2 (i.e. breathing) and where 50% of this exhaled used air (red) is recirculated back into the cabin (yellow), O2 and CO2 levels are poorer than in the cockpit not supplied with exhaled used air.


The AFA-CWA flight attendants association indicates how much fresh air each person receives:


  
  How much air is supplied in the economy section? 6-10 ft³/min of outside air to each person which is about half what is recommended in
  buildings and transportation vehicles.
  The cockpit gets 50-100 ft³/min per person of outside air, or sometimes a mix of outside and recirculated air depending on the
  aircraft type. That is up to 20 times more than in the passenger
  cabin.