We are required to calculate the mass of the gas at STP, we are given its volume.
Remember at STP:
1 mole = 22.4 L
volume = n x 22.4 L
We can use the above equation to first find the number of moles, then after we can find the mass. Covert the volume to litres.
V= n x 22.4 L
0.151 L = n x 22.4
n = 6.741x10^-3 mol
Now we have the number of moles and molar mass of O2 = 15.999 g/mol
m = 6.741x10^-3 mol x 15.999 g/mol
m = 0.108 g