Oak Park is a seemingly moderate-sized town that Bernard Walton and Eugene Meltsner passed through on their way to California in the episode #276: “Third Degree”. Here, Bernard was given a citation for going too slow on a highway, and eventually arrested. This was because he decided to testify against the ticket in the Municipal Court of Oak Park, since he thought that the events previous was part of a scam from the law. What happened was that he was found in contempt of the court and was sentenced to prison.
Evidently, most people in Oak Park speak in sophisticated words in conversation, like Eugene. A convention for these people is held in the person who introduced Eugene to the club, Bryant Jennings's, house.
After Bernard payed his fines, Oak Park is not heard from again, but plays an important role in Bernard and Eugenes' trip to San Diego.