According to documents published by KOA for their 10th and 20th anniversary celebrations, KOA began broadcasting on December 15, 1924. However, other web resources have shown an even earlier date. One popular historical resource, which is reportedly based on old official records, states that the station that eventually evolved into KOA was first licensed on March 20, 1922 to the Young Men's Christian Association (YMCA) on a frequency of 485 meters or 619 kHz.
The 1924 offices, studios, and transmitter for KOA were located in the building in the above photo located at 14th & Krameria in Denver. One of the two antenna towers is visible at the far right edge of the photo. These towers supported a horizontal wire antenna.
In 1932 a new transmitter site was constructed at what is now 18500 E. Colfax Avenue (at the intersection with Tower Road) in Aurora, CO. This is an old photo of the 1932 building and its 475 foot, 4-leg, self supporting tower. 
This section of a 1930s era aeronautical chart for the Denver area is part of an historic display at Denver International Airport. It shows the location of the KOA tower between Buckley Field and the old Sky Ranch airport east of Denver. The caption indicates that the tower topped out at 5,911 feet above sea level.
This photo on the back cover of the 1944 KOA 20th anniversary booklet shows that the Colfax Avenue tower was equipped with a large neon sign with the KOA call letters. It was also decorated with 15 squares of neon tubing spread along the length of the tower. This tower was top-loaded with a 'wagon wheel' type device and in the photo a circle of neon tubing can be seen surrounding the top of the tower. This must have looked awesome at night. The booklet states that this is "one of the highest self-supporting towers in the US." The official KOA QSL card for this era listed it as 475 feet in height.
This drawing in the 1934 KOA 10th anniversary booklet shows the station's intended primary coverage area at the time. The booklet also mentions affiliation with the NBC Red, Blue, and Orange networks.
In 1934 the KOA studios and offices moved into the NBC Building at 1625 California in downtown Denver. KOA occupied the 4th, 5th, and 6th floors. This building was also known as "Denver's Radio City."
This is a March 2002 picture of the Colfax Avenue transmitter building. It is presently home to the Region 1 headquarters of the Colorado Department of Transportation and the Colorado State Patrol. 
A marker located near the front entrance of the Colfax Avenue site declares that the building has been designated as an historic landmark by the City of Aurora. 
Close inspection of the building reveals many decorative stone inlays including at least two like this one. It depicts radio waves coming from what appears to be a Western Electric model 1A carbon microphone.
Another stone inlay directly above the polished steel front entry doors. It depicts radio waves spreading through the air from an antenna tower.
This area in front of the building was once the heat exchange pond for the cooling water that was circulated through the giant transmitter tubes. It is now filled with dirt.
The cross street here is appropriately named Tower Road and was named that because of the KOA tower. That's the former KOA building in the background of this photo. The old broadcast tower is long gone and a recent addition to the building sits where the tower was once located. A newer tower, visible on the left side of this photo, supports some of the current occupant's communications antennas. KOA transmitted from this site until 1959 when the transmitter was moved to its present location about 20 miles south of Denver in Parker, CO.
This is the transmitter building and the base of the 665 foot tower presently used by KOA. With 50,000 watts, a 5/8 wavelength tower, and a relatively clear frequency the station is widely heard. The area around the transmitter was very rural at the time this facility was built. However it is now one of the fastest growing regions in the US. The city of Parker has steadily grown around the transmitter site.
Today KOA is owned by Clear Channel Communications. The studios are co-located with 6 other Clear Channel stations in this $19 million dollar, 78,000 square foot, state-of-the-art broadcast facility on South Monaco Avenue in the Denver Tech Center West. The official KOA web site is at