Elvis’s “It’s Now or Never” owes a little bit to “O Sole Mio.”
I kind of remember that, except that he said–without irony–“I’d like to thank my COLLEAGUES Tchaikovsky, Prokofiev, Rachmaninoff …” He was never heard from again.
Here are a couple I haven’t seen mentioned yet, though they may not fall quite into the “pop/rock” category:
The musical Kismet had songs based on melodies by Borodin, such as "Stranger In Paradise."
An early Amy Grant song, “Sing Your Praise To The Lord,” has an intro (though not the main melody of the song) taken from Bach’s Well-Tempered Klavier.
Couldn’t find the exact quote, but it was Hans Zimmer, who won for his score for The Lion King in 1994. And he was, in fact, heard from again, though not by Oscar.
S Club 7 with “Natural” and Norma Ray with “Tous Les Maux D’Amour” (which “Natural” is just an English language version of) both use Gabriel Fauré’s “Pavane” as the basis for the song.
There are eight notes, plus accidentals. There will be some overlap.
And if you are going to steal, steal from the best.
This thread is incomplete without a reference to The Pachelbel Rant.
Done in post 7.
Well, now it’s twice as not-incomplete.
It does bear repeating.
But, it’s not nearly as terrific as SRC’s version.
And, if we’re talking cover versions of classical pieces, how about the spectacular Sabre Dance by Love Sculpture?
Radiohead’s “Exit Music: For a Film”, from OK Computer (and that movie with Diane Lane and Richard Gere), is based on (but not exactly the same as) Chopin’s Prelude in E minor.
(ETA: The version in the film is just on piano, so the connection is even more obvious.)
(ETA again: I think the ORIGINAL version was in a different film, that modern version of “Romeo and Juliet”.)
It seems like everyone has a cover of ITHOTMK 
BLACKMORE!
Dan Fogelberg’s Same Old Lang Syne uses the 1812 Overture, slowed down, as it’s melody.
[
](Same Old Lang Syne - Wikipedia)I heard an interview with Fogelberg years ago where he said that he HATED the 1812 Overture, but realized that slowed down it made a really good ballad.
(Also, color me surprised that Fogelberg died in 2007. That one slipped by me.)
From what I understand, Tchaikovsky hated it too.
This is great:
Adya & Geisha adapted from Mozart’s Cherubino’s Aria (from Marriage of Figaro)
Including Erasure
Before Elvis turned O Sole Mio into “It’s Now Or Never,” someone else (Perry Como, possibly?) recorded it as “There’s No Tomorrow.” His “Catch a Falling Star” and, believe or not, “Hot Diggety Dog, Ziggety Boom” were also based on classical themes. Likewise Della Reese’s “Don’t You Know” and Allan Sherman’s “Hello Mudduh, Hello Fadduh.”
So you never saw Mr Holland’s Opus?
In case you haven’t Alicia Witt becomes the governor
The more blatant rip-off was Graduation by Colleen Fitzpatrick.