Prince Oleg (Norse name Helgu) was the East Slavic ruler who moved the capital of Rus from Novgorod the Great to Kiev. He is thought to have been a supreme ruler from 879 to 912, although recently discovered Khazarian sources mention the activities of certain kagan HLGW of Rus ca 945.

A relation (likely brother-in-law) of the first ruler, Rurik, the Varangian Helgu was entrusted by Rurik to take care of both his kingdom and his young son Ingvar, or Igor. Oleg gradually took control of the Dnieper cities, captured Kiev (previously held by other Varangians, Askold and Dir) and finally moved his capital from Novgorod there. The new capital was a convenient place to launch a raid against Constantinople in 911 that won a favourable trade treaty, which eventually was of great benefit to both nations. The text of the treaty survives in the Primary Chronicle.

According to a legend recorded by Pushkin in his celebrated ballad, it was prophesied by the pagan priests that Oleg would take death from his stallion. He sent the horse away. Many years later he asked where his horse was, and was told it had died. He asked to see the remains and was taken to the place where the bones lay. When he touched the horse's skull with his boot a snake slithered from the skull and bit him. Oleg died, thus fulfilling the prophecy. In Scandinavian traditions, this legend lived on in the saga of Orvar-Odd. Two funerary barrows have been known as Oleg's graves, one in Ladoga and another in Kiev.