This paper analyses the diachronic study of Ogan language (Pegagan dialect) as one of the language in Indonesia and belongs to proto Austronesia. This language is spoken by people in Regency of Ogan Ilir (Ogan Komering Ilir/OKI), South Sumatera. By using diachronic study, this language is analyzed by using perspective of phonological and lexical form. This paper uses library research to analyze. Ogan language (Pegagan dialect) deserved to be studied because it has uniqeness, that it has lots of dialect and sub-dialect, compared with other languages in Sumatera island. The result showed that there are retention and innovation in Ogan language, especially Pegagan dialect. Furthermore, it can be concluded that Ogan language still retains its proto language but in the same time it has developed new vocabularies.