In typical Dylan fashion, "Oh, Sister" seems simple on the surface but becomes more complex the longer you look at it. The gist of it is that Dylan is accusing a woman he calls his sister of being cold to him.
"You should not treat me like a stranger," he says, because "time is an ocean but it ends at the shore" and she may not see him tomorrow. The whole implication is that life's too short to be petty and Dylan's reminding that he, like everyone else, can die at any time.
The song gets weirder and more Biblical, though. In the last verse, Dylan states that he and this mystical "sister" grew up together, died, were reborn and "then mysteriously saved." On the official Dylan website and in the official book release of his collected lyrics, the "Father" in the first verse is capitalized and so is the "His" in the second verse. This manner of capitalization is nearly always associated with a reference to God, which dramatically alters the meaning of the lines "our Father would not like the way that you act" and "is our purpose not the same on this Earth, to love and follow His direction"?
Looked at in this light, the "sister" may not be a biological sister at all. Some have said that it's actually Joan Baez and that the song is a reaction to Baez's "Diamonds & Rust," a song about their relationship.
The only real evidence for the Baez connection is that her song was released on here album Diamonds & Rust in April 1975, and Dylan started working on "Oh, Sister" a couple months later (June). Anything is possible, but it's most-often fruitless to look for literal, concrete biographical in Dylan's songs. Even when such things are actually there, Dylan buries them so deeply in metaphor and misdirection that you just get lost trying to find the way, though many would say "getting lost" is precisely the appeal of Dylan's music.
Whatever the ultimate meaning of the song is, "Oh, Sister" is considered one of Dylan's successes. It comes across as an authentic emotional appeal. The way Dylan's voice harmonizes with Harris' works very well, as does the interplay between his harmonica and Rivera's violin.
"Oh, Sister" was the B-side to the single "Mozambique." The lyrics were co-written with Jacques Levy, and Emmylou Harris sings harmony vocals. Scarlet Rivera plays violin. She joined Dylan on this album after Dylan saw her walking down the street with a violin case in hand. Having never heard her play a single note, he asked her to meet him at the studio.
The first time Dylan performed "Oh, Sister" live was in a Chicago TV studio in September 1975. He dedicated it to the legendary music figure John Hammond, who was Dylan's first producer. Dylan made the same dedication before playing it at several concerts. Hammond was still alive at the time (he passed in 1987), so it's not entirely clear what sparked this gesture so many years after Dylan's career began.
The first take of the song was recorded on July 30. It went through several takes before yielding the version included on Desire.