From the Ultimate Beatles Encyclopedia, under Ob-la-di, Ob-la-da:
 
The Beatles began recording this number, penned by Paul, on 3 July 1968
and continued for several days. The lighthearted song had been inspired
by reggae music and the number was originally intended to be a Beatles
single but John and George rejected it. The song was recorded by both
Marmalade and the Bedrocks in Britain and Marmalade topped the charts
while the Bedrocks reached #17 with their version. Ironically, it was
issued as a Beatles single in America years later on 8 November 1976. 
 
Alistair Taylor recalls an anecdote concerning the number. Paul received
a phone call on behalf of a reggae musician called Scott who was in
Brixton prison for failing to keep up with his wife's maintenance
payments. Apparently, Scott ran a band called Ob-la-di, Ob-la-da and
suggested that Paul may have got the title after seeing the name on the
band's posters in South London. If Paul could delivery him 111 Bpds to
have him released from Brixton, he'd drop all claims to the title. Paul
couldn't remember whether he'd ever seen a poster with that name on it,
but he felt sorry for the guy and instructed Alistair to take the money
to Brixton prison and arrange for Scott's release, which he did.