I assumed OG was 'Old Guy'.......
When I read this on my phone, I started laughing so hard in a meeting I had to take a break and spell it out.
I had been brought in to work with a 'hipster' start-up in 'Cali', and this 20-something started asking about my background, and then when I started the run down he kind of cocked his head, paused, then blurted out "D@mn Man you a Legit OG!"
As soon as I got a break I went looking for one of the founders, told him of the interchange, and asked him to translate. He laughed and then told me that it was a back-handed compliment. In his mind I was a legit "Original Gangsta" - seriously cracked me up that was a compliment.