The Orion Syndicate is a criminal organization created by the Orions. It has existed since at least the mid-22nd century and had grown to be a major interstellar criminal organization by the 24th century.
In 2370, Juraan was the leader of the Orion Syndicate in the Romulan Star Empire. His reign ended in 2385 when he was presumed executed. He would resurface in 2386, and reunite with the syndicate to bring about the end of the Federation-Romulan alliance. (Star Trek: Generation Fleet)
In 2375, Z'et a known Bandi that left Deneb IV, and used his skills to become a Syndicate chemist. He worked with the Son'a to make Ketracel-white. The Federation would shutdown the facility on Devos II, but he would escape. He later disguised himself as a chemist for the Lakat Corporation. It wasn't until 2389 that he surfaced on Starbase 140. (Star Trek: Generation Fleet)
In 2388, Hatcher would die at the hands of the Orion Syndicates who attacked his ship that was carrying Captain Candela Greene, Commander Teshurr, and Lieutenant Eric Kramer, as well as a shipment of Zentradi weapons. The move was made by Juraan for punishment in allowing a spy on the ship, and in the syndicate.(Star Trek: Generation Fleet; Star Trek Crusader)
In 2394, Syndicate leader Melanie D'ian, after taking command of the Syndicate in a coup, made a formal alliance with the Klingon Empire, and assisted in the invasion of Gorn space (Star Trek Online). As a result, a number of Orions entered the Klingon Defense Force, including Captain Mesla of the IKS Mish'ra. (Star Trek: Constance)