A single oxygen atom has eight protons, eight neutrons and eight electrons. The number of protons and electrons of an element are equal to its atomic number, which is eight for oxygen. The number of neutrons can be calculated by subtracting the number of protons from the atomic mass, which is 16 minus eight for oxygen.
An atom is primarily comprised of neutrons, electrons and protons. The neutrons and protons are contained within the nucleus, a tight central area of the atom. The electrons, which have a near-zero mass, exist in orbitals around the nucleus. Each orbital can only contain a certain number of electrons. Once a lower orbital is filled, electrons start filling a higher energy orbital.