It is a standalone word meaning "uncle" (specifically, father's brother or mother's brother).
Let's look at why it's not a suffix or prefix:
* Suffixes are added to the end of a word to change its meaning or grammatical function. "Oji" isn't added to the end of a word to change its meaning.
* Prefixes are added to the beginning of a word to change its meaning or grammatical function. "Oji" isn't added to the beginning of a word to change its meaning.
Examples of how "Oji" is used:
* Ojisan (おじさん): This is a polite term for "uncle."
* Oji-sama (おじさま): This is a more formal term for "uncle."
Let me know if you have any other Japanese words you'd like to explore!