This phenotype of OG Kush is believed to originate in the early 1990s in Southern California. It is perhaps a bagseed of the original OG, also known as Hell's OG or Tahoe OG. Then again, that's just one story on OG out of many legendary fables.
Also kown as Snoop's OG, this particlular phenotype is thought to originate in the mid 90's amongst the proliferation of West Coast hip hop. B-Real of Cypress Hill calls it Brain OG, probably after the song "Insane in the Brain", and Snoop calls it Snoop OG.