"Oldies" was a format of radio station, popular in the 1980's and 1990's. "Oldies" weren't truly a "genre" but instead were songs from a certain era (between about 1955 and 1970) with a certain feel (upbeat, catchy and short). Much of the appeal of Oldies radio was that their wide appeal and inoffensiveness was suitable for many settings, including the workplace. Their peppiness also were ideal for driving. The DJ's on Oldies station, much like the music, tended to be peppy and light.
Some of the genres played on Oldies stations included Doo-wop, British Invasion, Motown/soul/R&B, bubblegum pop, and rockabilly. An Oldies station might play The Beatles, The Supremes and Elvis Presley.
When I was a child and a teenager, Oldies stations were seen as kind of corny: what moms and dads listened to in their minivans. And while it is true that some of the songs played were the most formulaic and trite pop, they also managed to play a lot of quality music. Also, Oldies actually were more diverse than other formats on the radio: there was probably no other format that included as many African-American and female artists than Oldies radio.
Even at the age of 13 or so, I realized that Oldies radio was somewhat corny and outdated, but I have great memories of listening to The Beatles, The Loving Spoonful, The Ronnetts, Chuck Berry and many others on my one-speaker radio.