NOTE: I'll have to make the assumption that the conditions are standard temperature and pressure.
If this is true then
One mole of an (ideal) gas at standard temperature and pressure conditions occupies a volume of 22.41 "L".
We can use this to convert from liters of "O"_2 to moles (treating oxygen as an ideal gas). We need to convert from milliliters to liters:
5600cancel("mL")((1color(white)(l)"L")/(10^3cancel("mL"))) = color(red)(5.6 color(red)("L"
Using the above mole-liter conversion:
5.6cancel("L")((1color(white)(l)"mol O"_2)/(22.41cancel("L O"_2))) = color(blue)(0.25 color(blue)("mol O"_2
Now, we can use Avogadro's number (6.022xx10^23) to convert from moles to individual units (molecules) of oxygen gas:
color(blue)(0.25)cancel(color(blue)("mol O"_2))((6.022xx10^23color(white)(l)"molecules O"_2)/(1cancel("mol O"_2)))
= color(green)(1.5xx10^23 color(green)("molecules O"_2
