How many oxygen molecules touch you in your lifetime? Following up on a comment by BlueRaja to this beautiful answer of Ilmari Karonen, I would like to phrase this follow up question:
How many air molecules hit your average human's skin during their lifetime? Actually, make that how many oxygen molecules do you breathe in your lifetime?
(In case you want some motivation: Ilmari calculates that the chance for a random oxygen molecule in the atmosphere (though I note that local effects could actually play a role for this one) to have existed in that form since the time of Homo erectus is about one in 1014. However, there's a lot of them molecules, so maybe you do brush up with quite a lot of them often.)
Any takers?
 A: Analysis for Jesus's molecule usage:

Our breathing rate changes a lot, but on average its about 1 breath
  every five seconds, or 12 breaths a minute, or 720 breaths an hour, or
  17280 breaths a day or 6,307,200 breaths a year, and if we live for 32
  years that gives us 201,830,400 breaths in his lifetime. How many
  atoms? multiply 2.02e8 total breaths x 1.61e23 molecules per breath to
  get a total of 3.25e31 total molecules. ...That means for Jesus, there were
  32,500,000,000,000,000,000,000,000,000,000 molecules (325 decillion) that came into contact with his lungs during his lifetime.

I didn't see anything wrong with the author's approach, so say 6,307,200 * 1.61e23 molecules/year (based on 6 liters of air/breath not oxygen). Since you want $O_2$ only then scale that down.
At STP (Standard Temperature and Pressure: 1 atmosphere of pressure, 0C), one mole of any gas will occupy 22.4 liters of space. One mole of any substance contains 6.02 x 1023 particles. Air is about 21% oxygen making 1 liter of air to be about 0.21 liters of oxygen. 
To find out how many moles that represents=0.21 / 22.4.  Then use Avogadro's number 6.02 x 10^23 atoms/mole.  1 L = 5.64e21 atoms or 2.82e21 molecules/L.
6L/breath => 1.69e22 molecules/breath
6,307,200 breaths a year => 1.07E29 molecules of $O_2$/year.
How long are you going to live * 1.07E29 = $O_2$ passing into your lungs.
The only two caveats to this answer:  some of these molecules won't be unique and 6L represents gas going into the lungs, not what is utilized.
Say you live 80 years, that's ~8.5e30 molecules of $O_2$.
