The Duchy of Olasko is part of the Kingdom of Roldem in the fantasy world of Midkemia by author Raymond E Feist. The current Duke of Olasko is Varen Rodoski, who married the sister of Kaspar of Olasko, Natalia.
History
Olasko was once an independent nation, and one of the most influential of the Eastern Kingdoms. However, following the events that occurred in the novel 'King of Foxes' where Talwin Hawkins, agent of the Conclave of Shadows, leads his army to overthrow the Opardum (Olasko's capital city and home of Kaspar), Kaspar is banished and Talwin Hawkins hands Olasko over to the King of Roldem in order to stabilize the region.
References
Duchy of Olasko Wikipedia(Text) CC BY-SA