semilegendary Viking (Varangian) leader who became prince of Kiev and is considered to be the founder of the Kievan Rus state.

According to The Russian Primary Chronicle (Russian Primary Chronicle, The) of the 12th century, Oleg, after succeeding his kinsman Rurik as ruler of Novgorod (c. 879), went down the Dnieper River with his Varangian retinue and seized control of Smolensk and Kiev (882), which he subsequently made his capital. Extending his authority east and west of the Volkhov–Dnieper waterway, he united the local Slavic and Finnish tribes under his rule and became the undisputed ruler of the Kievan–Novgorodian state.

Described in the chronicle as a skilled warrior, Oleg defeated the Khazars, delivering several Slavic tribes from dependence upon them, and also undertook a successful expedition against Constantinople (907), forcing the Byzantine government to sue for peace and pay a large indemnity. In 911 Oleg also concluded an advantageous trade agreement with Constantinople, which regulated commercial relations between the two states and laid the basis for the development of permanent and lucrative trade activities between Constantinople and Kievan Rus.