You have to use the ideal gas law which is PV=nRT
P: pressure of the gas (in this case 1.2 atm)
V: volume of the gas (in this case 25 L)
T: Temperature of the gas (in this case 25 °C which is 298K)
(You need to convert all temperature values to Kelvin when using gas laws)
n: is the number of moles (in this case the unknown)
R: the gas constant which is 0.0821Latm/molK
rewrite the equation so that n=PV/RT than plug in all of the values.
You should get n=1.2262mol
Therefore the answer is 1.23 moles of oxygen.
I hope this helps. Let me know if anything is unclear.
