Language setting in toos gives option for Oriya (Odia). This uses inscript heyboard which is difficult to use. Keyman Desktop option will allow me phonetic input using roman script.
Character input is generally a feature of your operating system (which you did not specify). There isn’t much to do on the LibreOffice side.
From the keyman user;s guide (PDF):
After this, open Keyman and install the keyboard. You should be able to use the keyboard in a word
processor now.Load a text-editor or word-processor, such as Notepad, or WinWord.
LibreOffice is a word processor that will work like the others mentioned in the document.