What it means
An OG is an original, a respected veteran who was there from the start and earned their stripes, whether in a scene, a job or a friend group. Calling someone an OG is real respect: you are the real deal, the one who set the template the rest are following.
Usage examples
"He has been fixing bikes here since the eighties, total OG of the neighbourhood."
"You still use that first model? OG move, respect."
"Ask Dee about this estate, she's OG round here, been holding it down since day one."
"Nah, don't chat to him like he's new, that's an OG in the scene."
Where it comes from
OG comes from original gangster, a label that grew out of US hip-hop and street talk for somebody with real status, history, and stripes. Over time it broke out of that tighter gang and rap sense, and now people use it wider for anyone who's been there from the jump and still carries that earned respect.