In 1923 he went to work for manufacturer William Kenney as a mechanic. While there he began retrofitting legal slot machines to pay out in cash for Southside slot syndicate kingpin James "High Pockets" O'Brien. After leaving Kenney, Rockola continued his work for O'Brien. In1928, Anton Cermak was elected Mayor of Chicago on a promise to clean up the corrupt administrations of his predecessors. Cermak appointed John Swanson DA and in 1929 Swanson went after the slot machine syndicate with a passion. In June he formed a grand jury. In exchange for immunity, David Rockola agreed to turn state's evidence. During the trial, Rockola abruptly changed his mind and took the fifth (along with Kenney). He was charged with contempt and sentenced to six months in jail. The case against O'Brien (who was conveniently out of the country during the trial) fell apart but Chicago police officer and evidence room custodian Frank Beran was indicted for taking bribes to allow Rockola to take possession of confiscated slot machines. This time Rockola testified and Beran was convicted. Filled with a new confidence, the city decided to try High Pockets one more time. Once again, Rockola agreed to testify. Once again he changed his mind. Once again he was jailed and O'Brien got off scot free. After the trial Rockola began to sever his ties with the slot business and it organized crime associations.