Initially, you have
$4.80color(red)(cancel(color(black)("g O"_2))) × "1 mol O"_2/(32.00 color(red)(cancel(color(black)("g O"_2)))) = "0.1500 mol O"_2$
Thus, you have
$"0.1500 mol O"_2/"15.0 L" = "0.0100 mol O"_2/"1 L"$
If you now add 0.500 mol of $"O"_2$, you will have
$"(0.1500 + 0.500) mol O"_2 = "0.6500 mol O"_2$
Since volume is directly proportional to the number of moles, the new volume is
$0.6500 color(red)(cancel(color(black)("mol O"_2))) × "1 L"/(0.0100 color(red)(cancel(color(black)("mol O"_2)))) = "65.0 L"$