This study provides relatively detailed descriptions of inflectional morphology in the Oromo language. It identifies occurrence patterns of morphemes and draws rules for inflections in the language. Although it focuses basically on inflectional morphology, it in some ways, deals with derivational processes and syntactic structures for comparison and relational analysis. A degree of fusion of morphs and morphological occurrences of inflectional formatives have been thoroughly dealt with. The thesis describes the inflectional forms of, essentially, nouns and verbs. However, words occurring in the nominals and verb-related words occurring in the predicate position have also been examined. Since the two word classes (nouns and verbs) are mostly the ones that undergo inflection, they determine the inflectional characteristics of the language. The study consists of five chapters which are concerned, respectively, with introduction in which preliminaries and methodology are treated, literature review which deals with some related concepts and previous works on Oromo, nominal inflection (including nouns, pronouns and adjectives), verb inflection (including verbs and adverbs) and conclusion. Number, singulative, gender and case are considered in the nominal inflection. Verb inflection is described in terms of inherent and agreement properties of grammatical function. In the descriptive chapters, distribution of morphemes and their allomorphs, along with their hosts, have been examined. This study provides a relatively more comprehensive and detailed description of inflectional morphology in Oromo, and hence the research outcomes are more focused to forms and functions of inflections.