An OG Vibe is a remastered version of an original song that uses a beat similar(but not the original) to the original track it was recorded to.
This can be done to bring clarity to an old track, or done simply because the original music has not been released, lost.....ect.
Person 2: I agree, this unreleased Tupac track is great, but it's not the original.
Person 1: What do you mean?
Person 2: It's an OG Vibe.