DJ Sim is the alias of Dutch hardcore producer and DJ Marcel van Os. In addition to using the DJ Sim name, he has released music under various other aliases, including Charging Robot, Marcel van Os, Markos, Millennium Groove, No Velocity, Simtec, Simtec Power Squad, and Tommy 5 Tone. Many of his releases were produced during the peak of the hardcore scene in the Netherlands in the 1990s. Although he no longer produces hardcore music, Marcel van Os has shifted his focus to creating music for commercials. A distinctive feature of his work is the use of an "organ" sound in his melodies, which became a hallmark of his style during his active years in hardcore production.