Oyarsa is a fictional character in C. S. Lewis's Space Trilogy, which includes Out of the Silent Planet, Perelandra and That Hideous Strength. Oyarsa is the eldil--the 'god' (note the lowercase 'g') or archangel--who rules Mars (Malacandra) and its inhabitants--the pfifltriggi (singular pfifltrig), the séroni (singular sorn), and the hrossa (singular hross).
It should be noted that there are three Oyarsas named in the Space Trilogy:
Oyarsa of Malacandra and Oyarsa of Perelandra are the antithesis of the Oyarsa of Thulcandra, also called the "silent planet".
The plural of Oyarsa is Oyérasu.