A mole of molecules is simply a very large collection of molecules. In order to have one mole of oxygen molecules, you need to have exactly
This very large number, which is known as Avogadro's number, allows you to convert from moles of a substance to molecules of a substance.
In your case, the problem tells you that you have a total of
#1.26 * 10^(19) < 6.022 * 10^(23)#
To determine exactly how many moles of oxygen you have, use Avogadro's number as a conversion factor
#1.26 * 10^(19) color(red)(cancel(color(black)("molecules O"_2))) * overbrace("1 mole O"_2/(6.022 * 10^(23)color(red)(cancel(color(black)("molecules O"_2)))))^(color(blue)("Avogadro's number")) = color(green)(2.09 * 10^(-5)"moles O"_2)#
Alternatively, you can express this number in standard notation to get
#n = color(green)("0.0000209 moles O"_2)#
The answer is rounded to three sig figs, the number of sig figs you have for the number of molecules of oxygen.