‘Over the counter’ is a term that refers to transactions of buy and sell order that occur outside the ways of the stock exchanges, as Binance and Coinbase. Instead, the transaction takes place between two parties. The big advantage of this is that when large crypto numbers are sold via OTC it will not affect the market. A disadvantage is that an OTC transaction is not anonymous. After all, you know who the other party is.
Coinbase launched Prime at the end of 2018, to arrange over the counter transactions.