An OG (short for "original") username is a handle that was claimed early on a platform — early enough that it's short, clean, or a real word — before that platform started blocking new registrations of names like it. Because it can't be created fresh anymore, the only way to get one today is to buy it from whoever already holds it.
Every major platform eventually runs out of short, clean combinations and starts reserving or blocking them — partly to fight impersonation, partly because a 2-character handle is worth real money and platforms don't want to be in the business of handing them out for free. Once that happens, existing holders of those names become the only supply.
None of that is officially defined by any platform — it's just what the buyer/seller community has settled on as "OG."
Open the vault, pick a listed username, and DM the contact handle shown on the site to arrange payment — there's no cart/checkout, it's a direct conversation with the seller.
Crypto-only. See the Terms of Service for the full payment and delivery policy, including the sale-finality and 72-hour issue window.