in calendar terms mid-1950's through early 80's excluding Disco.
Why does it exclude Disco? If you're biased about those type of oldies, that means you should also exclude Motown, Soul, or Funk. Its all related. Rod Steward had a Disco song, so did Heart and the Stones.
Remember, you asked for my opinion.....so here it is.
Disco is dance music, pure and simple. It is to Rock/Pop what Swing is. Good in its own right but not within the genre.
As far as artists having crossovers.....Elvis sang some Standards, ballads and a whole bunch of Christmas songs. Does that mean everything he did deserves to be included in Rock/Pop? Of course not. The artist does not determine the song's genre, the song does.
I do not make a distinction between Motown, Soul and funk because they pretty much all sound the same, and although I am not personally a big fan of these, I have to acknowledge Motown, in particular, was a big part of Oldies at one time.