This paper discusses declension patterns and their application to selected oikonyms of Split-Dalmatia County. It also talks about the creative methods used in the formation of the demonyms and ktetic forms from the selected oikonyms of the Split-Dalmatia County. In the first part of the paper, a theoretical insight into the development of the study of these linguistic phenomena is presented, which is imbued with an onomastic and normative approach. 71 respondents participated in the test. The first group consisted of respondents whose place of residence is in Split-Dalmatia County, the second group consisted of respondents from other counties in the Republic of Croatia. Based on a questionnaire that consisted of 19 tasks, the knowledge of the declension of oikonyms and the formation of male and female demonyms and ktetics was tested. Also was tested the extent to which speakers of the Croatian language, to whom these oikonyms are spatially close, have difficulties in determining the grammatical categories of gender, number and declination type of oikonyms and the formation of demonyms and ktetics.Selected oykonms from the Split-Dalmatia County belong to different declension types and differ in terms of referentiality, clarity of appellative and complexity (single and multi-word oikonyms). The first part of analysis of the results included the declension of the selected oykonyms which were grouped according to gender, number and declension. In the second part were analyzed word-formation types used in the formation of male and female demonyms and ktetics. The purpose of this paper was to establish to what extent the given rules on the declension of oikonyms and the formation of demonyms and ketic forms are applied, and which factors affect their compliance with the norm. The presented results show that there are still many uncertainties and doubts about their use.