The notation of the isotopes using the atomic number and the mass number consists of the symbol of the atom, preceded by the mass number as a superscript and the atomic number as a superscript.
All the isotopes of the same element have the same atomic number. They only vary the mass number.
So, all the isotopes of oxygen have atomic number 8.
The isotope oxygen-16 has mass number 16, so it is written with the symbol O preceded by the number 16 as a superscript and the number 8 as a subscript (the two numbers to the right of the chemical symbol).
The isotope oxygen-17 has mass number 17, so it is written with the symbol O preceded by the number 17 as a superscript and the number 8 as a subscript.
The isotope oxygen-18 has mass number 18, so it is written with the symbol O preceded by the number 18 as a superscript and the number 8 as a subscript.
