Oleska (pron. uh-LES-ka) can be a diminutive form if the Russian name Olesya, spelled in Russian Îëåñÿ. According to this site
http://www.calend.ru/names/0/0/274/, it originates from the name
Alexandra.
The name Olesya has gained popularity upon a novel by
Alexandre Kouprine (
http://www.lib.ru/LITRA/KUPRIN/olesya.txt).