Old Gang of people... the original gangster. This doesn't seem wrong. OG directly translates to Original Gangster, but is used to refer to the old crowd, the original people, the firsts, etc. In extreme example, it would not be considered incomprehensible (but perhaps strange) to say something like "native americans are the OG north american inhabitants"... really nothing to do with gangsters.
Technically true, but... if you say it means "old group" or "original gaggle" or "oldest goat," you should probably expect to be corrected, because it sounds like an implication that that's what OG actually stands for.