[iszomer]

But what's considered as an OG account? Age? Short handle? Something that isn't imaginative or a portmanteau of something?

I remember the ICQ days where the shorter your ICQ number, you are the OG of OG's..

[Strom]

OG refers to precedence due to age. It also tends to correlate with short names, but that's just because people like short names - and so short names are registered earlier and thus are older.