Incredible composition. Verses start with what sounds like a chorus refrain (containing the title word), but this line links to a consequent line and then an open-ended third line, forming an odd, and uniquely short, verse structure. Short enough that they put two of these verses at the beginning, followed by what seems at first to be a bridge. So far, the song has been going back and forth between A minor and F, but this new section starts moving between A minor and B minor, suggesting (but not defining) a new tonal center. This is derailed, however, by a Bb minor chord, an extremely clever pivot back to the original progression.*
And in moving back to the original progression to end this section (so crucial to the composition that you can't really call it a bridge), we hear half of what had previously been the first line of the verse, followed by a concluding couplet (with vocal harmonies introduced for the first time) that turns out to be another refrain!
Substance of the song continues to expand with a third verse, but from here on out it's all lost-in-the-mystery repeat, first the second section again and then the first verse, softer and with an added vocal harmony part, and the second section one last time, keeping the upper harmony part going.
All of this framed by the haunting two word refrain only heard at the very beginning and the very end.
* Bb minor would be the borrowed minor iv chord if we look at F major as the home key. (A minor is really the home key, but this at least gives us a common way of defining and understanding the Bb minor chord.)
Oh-OK: The Complete Recordings CD