KOSI-FM signed on in 1968 airing an easy listening/beautiful music format. At the time, it was paired with KOSI 1430 AM. The stations were owned by Bill Armstrong, who would later become a Republican congressman and two term U.S. Senator. Armstrong sold KOSI-FM to Westinghouse Broadcasting in 1981. KOSI-AM then became KEZW in March of that year. In 1988, Westinghouse sold KOSI and KEZW to D&D Broadcasting, Inc. for $15.5 million.  By 1990, the format evolved into its current mainstream adult contemporary music format. Entercom would buy the station in 2002 from Tribune Broadcasting. Tribune owned the station from January of 1993 to December of 2002. From 2007 to 2011, KOSI aired the syndicated Delilah radio show. Delilah has since been replaced with local programming.
KOSI is the only radio station in the Denver area to switch its format entirely to Christmas music during the Christmas season. The change generally takes place during the week of Thanksgiving and continues until 12:00 AM on December 26, at which point the format goes back to soft adult contemporary. In 2008 the Christmas music continued until December 27. KOSI is not the only station in Denver to do this. AM 1430 KEZW plays exclusively Christmas music from just after Thanksgiving until December 26th, and plays several holiday songs daily until New Year's Day.