O is for Oxygen
Oxygen is an element necessary for metabolic processes of most life on Earth. It is also an important component of water, a necessary component of all life on this planet.
Oxygen has three stable isotopes, Oxygen-16 (most common), Oxygen-17 (very rare), and Oxygen-18 (rare). Typical mass spectrometers measure the ratios between Oxygen-16 and Oxygen-18, but recently studies including oxygen-17 have become common as instrumentation has improved.
Isotopes of oxygen are typically used to explore changes in temperature, especially for water-living animals, and can also be used to inform scientists about amounts and timing of precipitation and humidity for ancient environments on land.