I remember reading somewhere (on an old mailing list or message board) that Mike once made the comment that Brian got the idea of OMP from some old black guy who came into the studio and sang it. Don't remember the specifics, or if there were any other details.
It's in the Byron Preiss book, on page 62:
"Old Master Painter -- that was an old, obscure... black gospel sort of thing, sung by an old fellow that came by Brian's house one night. --Mike"