Recently I was more than a little surprised to see some music from the nineties described as "oldies", but then I thought about it and realized it really isn't that startling because the definition of oldies obviously varies with your age and background. Still, I think most of us would probably concede it was the baby boomer generation that first popularized the term, so maybe their preferences have the most legitimacy. (I say this speaking not as a boomer myself, but rather as a sort of "pre-boomer" — if there is such a thing.) I'd also guess most boomers' favorite oldies would probably be the music of the sixties.
When you get to a certain age, your memories can sometimes seem like bubbles rising to the surface of a tar pit - slow and labored in transit, and often accompanied by a suspicious aroma. I think that might be what happens when I first try to remember how music was in the sixties because all I can think of is the Beatles. However, if I dig a little deeper I remember it was actually one of the most richly varied periods in pop music. In addition to the popularity of the "mop-tops" and their copycats, we were experiencing the evolution of early rockabilly and doo-wop into variations that ranged from bubble-gum pop to the Motown sound, while at the same time California was weighing in with surfing tunes, folk music, and the psychedelic sound of the "hippies" beginning to - er - flower in San Francisco.
The various California sounds provided the background for the rise of a singing group that exploded into pop music in the sixties and became a case study for everything good - and bad - happening in music at the time. The Mamas and Papas were a huge success musically with their "sunshine pop" mega-hits but their lives contained an almost endless amount of turmoil and trouble, with drug and sex scandals of a frequency and variation to almost strain credulity. The trials and tribulations of the members of the group have been well-documented and you can find all the stories you want by doing some searches on the web, so I'll only say the whole saga would probably make a great movie — except it might not be believable. (Although I guess that wouldn't necessarily disqualify it.)