Jefferson County Open School (JCOS) is the original public option school in Jefferson County and is located in Lakewood, Colorado.

In 1969 the Jefferson County School's Board of Education responded to the needs and interest of a group of parents and students to establish a learning community which honored the development of the whole child.

For over forty years, the Open School has provided a viable, vibrant, and life-changing alternative to conventional schooling. JCOS features a pre-Kindergarten through 12th grade environment with multi-aged groupings throughout the school. Students at all levels of the program work closely with their family and Advisor to develop highly personalized curriculum. Students are supported by their Advisor to take the personal, social, and intellectual risks necessary to discover the joy of lifelong learning.

The first class of students was welcomed into the Open Living School in Arvada in 1970. Due to overwhelming success and interest, two additional campuses were added the following year in Edgewater and Evergreen. In 1975 Mountain Open High School was added to the Evergreen campus, making Evergreen a pre-Kindergarten through 12th grade program. Eventually, the elementary programs were unified in Golden as Tanglewood Open Living School, while the high school remained in Evergreen. In 1989, Tanglewood and Mountain Open were reunited as a pre-Kindergarten through 12th grade program at the current location in Lakewood.