Oxygen, after hydrogen and helium, is the most abundant element in the universe. It is a fundamental tracer to learn more about the formation of single stars as well as entire galaxies. Understanding the origin of highly excited states of oxygen in the circumgalactic medium (CGM) around galaxies has proven difficult, and past theoretical models have had difficulty matching observational constraints. Using cosmological simulations from the IllustrisTNG suite, researchers at MPA have demonstrated how the feedback from supernovae and supermassive black holes can shape the heavy element content of the CGM, bringing it into agreement with data from the local universe. The amount of highly ionized oxygen around blue, star-forming galaxies is predicted to be noticeably higher than around red, quenched systems of exactly the same mass.