Oxygen is the third most abundant element in the Universe and the most abundant element of the terrestrial planets. The presence of oxygen in gaseous, ice and dust phases makes oxygen isotopes important tracers of various fractionation processes to form a protoplanetary accretion disk, which are essential for understanding the evolution of building blocks for planet formation. Photodissociation of CO isotopologues in cold interstellar environments forms H₂O ice with depletion of ¹⁶O component relative to the interstellar CO, but with heritage ¹⁷O/¹⁸O ratio from the interstellar CO. Dynamic evolution of protoplanetary disk generates H₂O enrichments inside snowline of the disk to change from ¹⁶O-rich to ¹⁶O-poor gaseous environments. Thermodynamics during heating processes reset oxygen isotopic compositions of dust in the disk to the gaseous oxygen isotope values. Therefore, building blocks of planet show oxygen isotope variations with variable ¹⁶O component, but with similar ¹⁷O/¹⁸O ratio each other. Oxygen isotopic compositions of outer planets would be ¹⁶O-poor in order of increasing distance from the Sun if outer planet formation started from icy planetesimal accretion.

Atmospheric oxygen evolution has long been discussed, especially with its relevance to the origin and evolution of life and the planet. Presence/absence of detrital redox-sensitive minerals, iron formations and red beds, behaviors of redox-sensitive elements in paleosols (ancient, subaerially-altered continental rocks) and ratios of carbon and sulfur stable isotopes in sedimentary rocks have been utilized to constrain atmospheric oxygen levels, which can dictate surface redox states, in the distant past, leading to a conventional view that the beginning (2.5–1.8 Ga) and ending (0.8–0.5 Ga) of the Proterozoic were two major periods when the oxygen level significantly increased in the Earth's history. More recent studies adopt multiple sulfur isotopes, iron speciation and trace elements (isotopes) as additional redox proxies. These proxies are not inconsistent with the conventional view, but the magnitude and timing of changes in these proxies are different between proxies and between geological records obtained from, e.g., iron formations, shales and paleosols. Also, the proxies suggest that there may have been transient oxygen increases of uncertain magnitude at 3.3–3.0 and 2.7–2.5 Ga. To better understand atmospheric oxygen evolution, process-based methods which quantify oxygen levels from individual proxies need to be developed to consistently and comprehensively explain multiple geochemical signatures.