We can calculate the amount of Oxygen in this volume using the Ideal Gas Equation, PV = nRT, where "P" is the pressure, "V" is the volume, "n" is the number of moles of material, "R" is the gas constant, and "T" is the temperature in Kelvin. To properly answer this problem, all of the information needs to be converted into the proper units. Fortunately, everything except the volume and temperature is in the correct units for the Ideal Gas Equation; the volume can be adjusted by converting mL to L (x1000) and the temperature can be adjusted by adding 273.15 to the current temperature (conversion from Celsius to Kelvin).
Plugging in all the values, we find that:
PV = nRT
(2.7 atm)(0.3 L) = n(0.0821*)(313.15 K)
n = 0.0315 mol Oxygen
Under these conditions, 0.0315 moles of oxygen can be placed in this volume.
Hope this helps!
* - the units are liters times atmospheres divided by moles times Kelvin.