OTC differs from trading on an Exchange. A trade happens directly between two parties, with one of those parties typically being a “desk” — a business dedicated to the buying and selling of a particular asset class. In an OTC trade, two parties agree on a price and then work out the transfer of assets between themselves.
Crypto OTC is simply the trading of crypto assets directly between two parties. A trade can be crypto-to-crypto (swapping Bitcoin with Ether for example) or fiat-to-crypto (swapping US dollars for Bitcoin and vice versa).