Oxygen-17 (O) is a low-abundance, natural, stable isotope of oxygen (0.0373% in seawater; approximately twice as abundant as deuterium).
As the only stable isotope of oxygen possessing a nuclear spin (+5/2) and a favorable characteristic of field-independent relaxation in liquid water, O enables NMR studies of oxidative metabolic pathways through compounds containing O (i.e. metabolically produced HO water by oxidative phosphorylation in mitochondria) at high magnetic fields.
Water used as nuclear reactor coolant is subjected to intense neutron flux. Natural water starts out with 373 ppm of O; heavy water starts out incidentally enriched to about 550 ppm of oxygen-17. The neutron flux slowly converts O in the cooling water to O by neutron capture, increasing its concentration.  The neutron flux slowly converts O (with much greater cross section) in the cooling water to carbon-14, an undesirable product that can escape to the environment:
: O (n,α) → C
Some tritium removal facilities make a point of replacing the oxygen of the water with natural oxygen (mostly O) to give the added benefit of reducing C production.