You have got an oxygen ATOM, for which #Z=8#. And what is #Z#? It is the #"atomic number"#, the number of massive, positively charged particles that comprise the oxygen nucleus. The oxygen nucleus also contains NEUTRONS, massive particles of ZERO charge. Interactions between protons and neutrons, at impossibly short nuclear ranges, constitute the strong nuclear force, an attractive force that is strong enuff to overcome the electrostatic force of repulsion between particles of LIKE electric charge.
Now there may be various numbers of neutrons within the atom of an element. And this gives rise to the phenomenon of isotopes; i.e. #""^16O#, #""^17O#, #""^18O#....all of these are oxygen atoms, and so they ALL have 8 nuclear protons, BUT they have #8#, #9#, and #10# neutrons respectively......giving rise the superscript that is added to the atomic symbol....
For garden variety #""^16O#, which accounts for 99% of oxygen, there are necessarily the 8 neutrons.....
