Whether or not a word/name is 'OG' (Originally meaning Original Gangster but has undergone a redefinition within the Minecraft community) is subjective, but typically what someone means when they someone has an OG name is that the word is in its basic form (the base form of a verb is the most simple form of a verb, without a special ending).
A few examples of words in base form are Game, Point and Financial.
You may also have heard the term 'Semi' or 'Semi OG' meaning a word/name is partly OG. A 'semi' name is typically a derivative (a word formed from another word or base), such as Gamers, Pointed or Financially.
Hope this answers your question.