We see that one formula unit of #"KClO"_3# contains three "O" atoms. So,
#bb("1 mol KClO"_3 = "3 mol O atoms")#
∴ #"Moles of O atoms" = 0.1004 color(red)(cancel(color(black)("mol KClO"_3))) × "3 mol O atoms"/(1 color(red)(cancel(color(black)("mol KClO"_3)))) = "0.3011 mol O atoms"#
Step 4. Calculate the number of #"O"# atoms
#"No. of O atoms" = 0.3011 color(red)(cancel(color(black)("mol O atoms"))) × (6.022 × 10^23 color(white)(l)"O atoms")/(1 color(red)(cancel(color(black)("mol O atoms"))))#
#= 1.81 × 10^23color(white)(l)"O atoms"#
Thus, #"12.3 g of KClO"_3# contains #1.81 × 10^23 color(white)(l)"O atoms"#.
Here is a video on converting moles of a compound to atoms.