The Oans, most often referred to as Guardians of the Universe, are a near-immortal alien species which originated on planet Maltus before migrating to Oa. Today they are most well known as the founders and administers of the Green Lantern Corps.
A phenomenally ancient species (the first sapient species to evolve, according to some), Oans are small-sized humanoids characterized by a large head, blue skin and white hair. Their blood is yellow and their hearts are green. Their civilization has evolved up to the point of them becoming almost god-like figures, biologically immortal and extremely wise. Though sometimes arrogant, they are mostly peaceful, and take along responsibility to help to protect the galaxy and its younger races.
Billions of years ago, however, it appears that they were far more careless, as an awry scientific experiment performed by them in an attempt to observe the beginning of the Universe accidentally resulted in a vast introduction of entropy into the primordial atom, causing the Universe to "be born old". After this, the Maltusians as they were called then; relocated themselves to the planet Oa at the center of the Universe. Other of their experiments were responsible for the evolution of the Psion species; and for splitting the primitive Martians into two distinct species: Green and White Martians. They were also the ones who guided Kal-El, the last of Kryptonians; in his journey to Earth to become Superman.
The Oans' racial abilities include telepathy, telekinesis, near-immortality, and the power referred to as the Glow, which grants them with the ability to manipulate the energy associated with the willpower of all living things. One of their only known weaknesses is antimatter energy. Although benevolent, the majority of Guardians are known to display conservative, bureaucratic tendencies as well as a Vulcan-like tendency to prefer cold logic over emotion. Throughout history they have fought and imprisoned many evil or undesirable entities. The Guardians have access to one of the most powerful energy sources in the Universe: the green element; and have used it to construct the Green Lantern rings and the Central Battery that charges them.
The ancient Maltusian species has given rise to at least two other species besides the Guardians: the Controllers were rebels who believed that the only way to effectively protect the Universe was to be in complete control of it. The near-human Zamarons are descendants of the female members of the species; since once they became immortal reproduction was no longer necessary the male and female diverged from each other. Furthermore, a group of Oans were once exiled on Earth and gave rise to the Leprechaun legends.