DJ Sim is the alias of Hardcore Producer and DJ Marcel van Os. He also produces under the name of Charging Robot, Marcel van Os, Markos, Millennium Groove, No Velocity, Simtec, Simtec Power Squad and Tommy 5 Tone. Most of releases were made during the height of the hardcore craze in the Netherlands in the nineties, he no longer produces hardcore music and now makes music for commercials. Most of his releases use his trademark "organ" sound for the melodies.