:Ok, I reread several chapters from Kostomarov myself. If anyone doubts that the word "oppression" is appropriate, I recommend reading the chapters from his book devoted to the following historic figures available online: Bohdan Khmelnytsky , Konstanty Wasyl Ostrogski ,  Peter Mogila. Not only Kostomarov is known for his views at times deviating from "official" line of thought in the RU Empire. The conclusion I make here, is based not on his opinions, but by quotes he makes from official documents and the older chronicles. I could find his book online only in Russian, sorry. Additionally, I recommend the following chapters from modern Britannica's "History of Ukraine" article:  
 *Lithuanian and Polish rule  
 **Social changes  
 **Religious developments  
 *The Cossacks  
 **The Khmelnytsky Insurrection  
 **The Ruin  
 **The autonomous hetman state and Sloboda Ukraine  

 If EB's "History of UA" article isn't enough, I can recommend EB's "History of PL" article, particularly the following subsections of "The Commonwealth" chapter: "Wladyslaw IV Vasa", "The Cossacks", "Bohdan Khmelnytsky". I happen to have full access to EB and these are just a couple of quotes:  
 :"...Ukraine was “colonized” by both Polish and Ukrainian great nobles. Most of the latter gradually abandoned Orthodoxy to become Roman Catholic and Polish. These “little kings” of Ukraine controlled hundreds of thousands of “subjects”... The new Eastern-rite church became a hierarchy without followers while the forbidden Eastern Orthodox church was driven underground. Wladyslaw's recognition of the latter's existence in 1632 may have come too late. The Orthodox masses—deprived of their native protectors, who had become Polonized and Catholic—turned to the Cossacks." 
 it goes further: 
 :" The heavy-handed behaviour of the “little kings,”... was resented even by small nobles and burghers. Growing socioeconomic antagonisms combined with religious tensions."'' 
 Can we return "oppression" in view of this?