Oleg of Novgorod (Old East Slavic: Олег, Old Norse: Helgi) was a Varangian prince (or konung) who ruled all or part of the Rus' people during the early 10th century.

He is credited by Rus' Chronicles with moving from either Aldeigjuborg or Novgorod the Great, and seizing the power in Kiev from his brother Askold, and by doing so, laying the foundation of the powerful state of Kievan Rus'. He also launched at least one attack on Constantinople, capital of the Byzantine Empire. According to East Slavic chronicles, Oleg was supreme ruler of the Rus' from 882 to 912.

This traditional dating has been challenged by some historians, who point out that it is inconsistent with such other sources as the Schechter Letter, which mentions the activities of certain khagan HLGW of Rus' as late the 940s, during the reign of Byzantine Emperor Romanus I. The nature of Oleg's relationship with the Rurikid ruling family of the Rus', and specifically with his successor Igor of Kiev, is a matter of much controversy among historians.

According to the Primary Chronicle, Oleg was a relative (likely brother-in-law) of the first ruler, Rurik, and was entrusted by Rurik to take care of both his kingdom and his young son Igor. Oleg gradually took control of the Dnieper cities, seizing the power in Kiev by tricking and slaying Askold and Dir, and naming Kiev the capital of his newly created state Kievan Rus'. The new capital was a convenient place to launch a raid against Constantinople in 907.

...
Wikipedia