OTC is an exchange between fiat money and BTC or ETH. exchange local services only offer as low as $ 30k - $ 40k at a time. But if you want to trade with large volumes, you should use Huobi global. Although Binance also has OTC service, users are not as crowded as in Huobi Global. you can buy and sell at a better price there but have to do KYC with a high level.