Answer by Peter Levy:
To record the vocal for this song, Paul went into the studio each day for a week and recorded exactly one take of the lead vocal. It’s such a strenuous song to sing that he couldn’t try two times in the same day.
He also said at the time (when he was in his mid-twenties) that he could have nailed the vocal five years earlier. So he’s had trouble with the vocal from the very start.
I think that’s why he didn’t sing it in concert – it’s too hard.
These days he has trouble with a lot of vocals that he used to easily perform in concert, so it’s fair to say that he never will perform “Oh! Darling,” unless he changes the arrangement so it fits his range.
Answer by Jay Snead:
t’s a killer song to sing the way it was sung on the album. This was really hard for Paul to record even when he was 27 and on the top of his game.
If he did it early in a set, it would ruin his voice for the rest of the show. if he sang it last, he might not have the voice to pull it off.
He has an enormous repertoire and can afford to leave Oh! Darling alone.
Answer by John Nowman:
Paul went into the studio every day – over days and days in order to get his vocals sounding hoarse, which is the effect he wanted and had to nail the vocals, when finally put down, quickly before his voice let him down due to strain and effort. Lennon had similar vocal problems with Twist and Shout early in their career, as the track was left until the end of the recording session, where he had to produce the vocals in one go to prevent damage to his his vocal chords, He did it in 1 and 1/2 takes, I believe. Listen to the track and see how hoarse his voice actually was after a full day recording vocals for the LP they were working on – at this moment early in their career time and money were not a luxury then. On Revolution Lennon lay on his back for hours prior to singing the vocals allowing fluid to enter his lungs for the right sound effect. Hope this helps.
Read more: (5) Why has Paul McCartney never performed Oh! Darling live? – Quora