"Where do exchanges OTC desks find OGs to get supply from?" For starters the exchanges know who has a lot of coin because they take their deposits, same way a bank knows their wealthy clients. They create relationships. They encourage large entities to use the desk instead of live books. And if you're an OG sitting on thousands of coins, you might welcome the opportunity to minimize slippage on a big profit take where you'd otherwise nuke the exchange or have to spread your orders across time. A huge buyer showing up creates opportunity for large holders who might otherwise be illiquid purely because of their size. This isn't all that different from dark pools in tradfi. Just some thoughts.