Radio stations that specialized in rock music recorded during the later 1960s, 1970s, and 1980s were generally labeled Album-Oriented Rock (AOR) stations. The symbiosis between AOR stations and bands such as Deep Purple, Led Zeppelin, and Aerosmith has led many to refer to virtually all 1970s era hard rock bands as AOR as well. When it was first introduced in the late 1960s, the AOR format was only marginally commercial, but by the mid-1970s AOR stations were taking on many of the characteristics of top-40 stations. As the popularity of AOR stations grew, major label record companies exerted increasing influence over AOR playlists around the country, in the process squeezing out competition from independent label competitors. A by-product of this influence peddling was a creeping homogenization of rock music available on radio stations.

The AOR format was happened upon after the Federal Communication Commission (FCC) mandated a change in the way radio stations did their business in 1965. The FCC prohibited stations from offering the same programming on both AM and FM sides of the dial. This ruling opened the less popular FM side of the dial to a variety of less commercial formats, including jazz and classical. Coincidental with this change in radio programming law was the emergence of the so-called "concept album" among British art rock bands, the Beatles, and Bay Area psychedelic bands. Some of these albums featured songs substantially longer than the three minute time limit traditionally observed by radio station programmers. In areas with massive collegiate populations, especially San Francisco, a few FM stations began playing entire album sides. This approach to radio programming departed significantly from the singles-only AM pop rock format.

Album-oriented rock

The roots of the Album-Oriented Rock (AOR) radio format began with programming concepts rooted in 1960s idealism. The Freeform or Progressive formats developed the repertoire and set the tone that would dominate AOR playlists for much of its heyday.

In the mid to late 1960s, the U.S. Federal Communications Commission (FCC) enacted a non-duplication rule prohibiting FM radio stations from merely running a Simulcast of the programming from their AM counterparts. Owners of AM/FM combo stations fought these new regulations vigorously, delaying the new rules for eighteen months. When finally enacted, station owners were pressed to come up with alternate programming options quickly.

The Freeform format in commercial radio was born out of this desperate need to program the FM airwaves, inexpensively. Programmers like Tom Donahue at KMPX developed stations where DJs had freedom to play long sets of music, often covering a variety of genres. Songs were not limited to hits or singles; indeed the DJs often played obscure or longer tracks by newer or more adventurous artists than heard on Top 40 stations of the day. This reflected the growth of albums as opposed to singles as rock's main artistic vehicle for expression in the 1960s and 1970s.

With a few exceptions commercial Freeform had a relatively brief life. With more and more listeners acquiring FM radios, the stakes became higher for stations to attract market share so that they could sell more advertising at a higher rate.

By 1970 many of the stations were moving to institute programming rules with a "clock" and system of "rotation". With this shift, Stations formats in the early 1970s were now billed as Progressive. DJs still had much input over the music they played, and the selection was deep and eclectic, ranging from folk to hard rock with other styles such as Jazz fusion occasionally thrown in.

A broad cross section of rock music that gained popularity during this time came to be called Progressive rock, likely because the wide recognition and success of artists could be attributed to airplay on Progressive stations; much the way the College rock label was given to bands that received air play on student-run college stations during the 1980s.

Album-oriented rock

In the mid-1970s, as program directors began to put more controls over what songs were played on air, Freeform and Progressive stations evolved into the album-oriented rock format. Stations still played longer songs and deep album tracks (rather than just singles), but program directors and consultants took on a greater role in song selection, generally limiting airplay to just a few “focus tracks” from a particular album and concentrating on artists with a slicker-produced, "commercial" sound than what had been featured a few years earlier. Noted DJ "Kid Leo" Travagliante of influential station WMMS in Cleveland observed the changes in a 1975 interview: "I think the '60s are ending about now. Now we are really starting the '70s. The emphasis is shifting back to entertainment instead of being 'relevant'...In fact, I wouldn't call our station progressive radio. That's outdated. I call it radio. But I heard a good word in the trades, AOR. That's Album-Oriented Rock. That's a name for the '70s".^[1]

The "rock" in album-oriented rock came in the late 1970s, when AOR music libraries and playlists discarded the wide range of genres embraced earlier on to primarily focus on a rock-centric sound. The occasional folk, jazz, and blues selections became rarer and most black artists were effectively eliminated from airplay. Where earlier soul and R&B artists like Stevie Wonder, War, Sly Stone and others had been championed by the format, AOR was no longer representing these styles^[2], and took a stance against disco. In 1979, Steve Dahl of WLUP in Chicago destroyed disco records on his radio show, culminating in the notorious Disco Demolition Night at Comiskey Park. Steve Slaton of KISW in Seattle had a similar on-air bit which was included on the station’s Epic Rock record album, as did Jay Preston of WLBJ in Bowling Green, Kentucky, who would play a few lines of a current disco hit, then unceremoniously run the stylus across the record numerous times and break the record on the air.

What links the Freeform, Progressive, AOR and ultimately the Classic rock formats are the continuity of rock artists and songs carried through each phase. Programmers and DJs of the Freeform and Progressive phases continued to cultivate a repertoire of rock music and style of delivery that were foundations of AOR and now Classic rock. Those AOR stations, which decided to stay "demographically-rooted", became classic rock stations by eschewing newer bands which their older listeners might tune out. Those that didn't fully evolve into classic rock generally attempt to hold onto their older listeners through careful dayparting—playing large amounts of classic rock during the 9-5 workday with more adventurous, newer songs "baked into the mix" as the listener base skews younger at night.