The doctoral thesis presents the analysis of the present-day Vilnius County toponym oppositions, their formation, etymology, and semantics, based on the integration of the scientific-theoretical principles and methodologies of traditional and cognitive onomastics, developed by Lithuanian and foreign scholars, as well as theories of cognitive linguistics and cognitive onomastics, and traditional onomastics research models. Oppositions, which are one of the means of categorizing the world, are viewed not only as antonymic semantic relations between the differentiating markers of toponyms, but also as any relationship between them, including elements of compound toponyms that are not semantically charged, but perform a differentiating function in toponymy. The combination of these theories and research principles together with the interpretation of extralinguistic data helped to expand the possibilities of linguistic analysis of toponyms and in most cases if not helped to restore the original meanings of toponyms, then to at least clarify their etymologies. Toponym oppositions are characteristic of toponymy in various territories. Thus, in this work, the assumption is made that such nomination models have not yet received due attention in Lithuanian onomastics. They are not only significant in marking geographical objects in the territory of Lithuania but also play a key role in the formation of the national onomasticon, because such formal and semantic sequences of toponyms are made taking into account both linguistic and extralinguistic conditions in the territory under investigation. It became clear that the analysis of structural-semantic toponym oppositions helps both to identify many motivating factors that determined the origin of toponyms and to reveal the principles of naming toponymic units. The investigation has shown that toponym oppositions are complex linguistic structures characterized by at least two conceptual levels, i.e., the conceptual structure of differentiating markers and that of identical toponym lexemes. The opposition in toponymy is influenced by extralinguistic factors and is determined by the needs of the nominator to express the relationship of new names to existing ones by linguistic means.