Русско-англо-русский юридический словарь

Bonds can be traded over-the-counter (OTC) and, in fact, the majority of corporate bonds that are issued by private and public corporations are traded OTC rather than on exchanges. Furthermore, many of the transactions involving exchange-traded bonds are done through OTC markets. (Investopedia)

An OTC or over-the-counter transaction, facilitates trades between two parties more privately, with one party acting as a “desk” (the business involved in buying and selling a particular form of an asset). The two parties agree on a price and then work out the transfer of assets between themselves. (Business 2 Community)