To solve this type of problem, you would want to use the combined gas law equation PV = nRT where:
"P" is the given pressure in KPa
"V" is the volume in L
"n" is the number of mols
"R" is the gas constant (Always 8.31)
"T" is the given temperature in K
In your example, you are told that the gas is at STP. This means that the pressure is 101.3 KPa and the temperature is 273K. Also, you know that the volume is 10.0L. Knowing all these values, just plug them into the equation above and solve for the number of moles, "n".
(101.3)(10.0) = n(8.31)(273) --> Now, divide by (8.31)(273) on both sides to get "n" by itself!
(101.3)(10)/(8.31)(273) = n
n = 0.4465 mols
Therefore, 0.45 mols of oxygen gas are present!
Hope this helps! :)