Cow killer!
Nope, it definitely was Tommy Johnson. The later, and more successful Robert Johnson traded on this confusion, but the Coen brothers got it right. Tommy was recording in the late 1920s, and Robert in the late 1930s.
The thing that really sells the line is George Clooney’s grin while delivering it. While he’s more on the “Movie Star” side of the “Actor to Movie Star” range, he displays hilarious comic timing and delivery in this film.
The actor is dressed like that picture of Robert Johnson in the Wiki article, down to the same tie. 
I’m talking about the legend of Robert Johnson–who may have stolen the story from Tommy. Who probably stole it from some other bluesman.
As I pointed out, Pappy O’Daniel was actually a Texas governor. And, as an adaptation of The Odyssey, the film was pretty non-linear…
One version is that Johnson took a year off and practiced; Son House began the rumor that Robert had sold his soul.
This is a funny story:
I thought George Clooney was channeling Clark Gable in this movie. His long winded, overconfident speeches and suave but thickheaded demeanor remind me a lot of Gable in It Happened One Night.
I wouldn’t put it past the Coen brothers to sow confusion. Or the costume person was thinking of Robert Johnson. But it’s definitely Tommy Johnson in the film for a reason.
“If you want to learn how to play anything you want to play and learn how to make songs yourself, you take your guitar and you go to where a road crosses that way, where a crossroad is. Get there, be sure to get there just a little 'fore 12:00 that night so you’ll know you’ll be there. You have your guitar and be playing a piece there by yourself… A big black man will walk up there and take your guitar, and he’ll tune it. And then he’ll play a piece and hand it back to you. That’s the way I learned to play anything I want.” - Tommy Johnson
I’m 550 miles from my home and Robert Crumb’s biography of Tommy Johnson. But here’s some information.
Tommy Johnson first recorded in 1928. Robert Johnson in 1936. The movie was set in 1937. Make of that what you will.
Given what the Coens did with “The Odyssey,” any presumption of sticking to the facts of any particular musician (or bank robber) is out of the question.
“You ruined my life!”
“I do apologize about that Pete.”
Does anyone know who sings the version of “I’ll Fly Away” which we hear in the film? The soundtrack version isn’t nearly as good.
I’ve seen that attributed to Robert, although it’s not the insert of The Complete Recordings as I thought. 
Thanks for the link.
For a time, the San Francisco Giants, whenever they would come to bat in the bottom of the 9th inning and trailing, would run a video clip on the scoreboard of George Clooney saying “Damn! We’re in a tight spot!” Just one of many reasons to love the Giants.
I always thought Tommy Johnson was just a thinly disguised pseudonym for Robert Johnson. Ignorance fought.
And I think the Allison Krauss “Down to the River to Pray” on the soundtrack (which was originally “Down to the Valley to Pray”, btw) is so beautiful it brings tears to my eyes.
#1 on my list of all time favorite movies. If it’s on TV, everything in the house stops.
+1 who thinks it’s a good movie and a good sound track.
**Pete: **Wait a minute. Who elected you leader of this outfit?
Ulysses Everett McGill: Well Pete, I figured it should be the one with the capacity for abstract thought. But if that ain’t the consensus view, then hell, let’s put it to a vote.
**Pete: **Suits me. I’m voting for yours truly.
Ulysses Everett McGill: Well I’m voting for yours truly too.
[Everett and Pete look at Delmar for the deciding vote]
Delmar O’Donnell: Okay… I’m with you fellas.
Clooney’s double-take at that response is something to behold, a thing of beauty.
O Brother … I’ve been away traveling, and this is my first post, post-trip.
It took me a loong time to “get into” this film, but when I did - !!!
Thanks to all the smart folks who pointed out the Tommy Johnson/Robert Johnson references. The whole film is so redolent of The South, the region with which I have a true love/hate relationship.
This is my favorite movie ever. Agree with Equipoise about its eminent quotability.
“They sang into yonder can and skedaddled.”
It sounds like the film version is also Allison Krause and Gillian Welch, same as the CD soundtrack. But I do wish I could get hold of the film version.
It’s not. I tracked down the film version, which is sung by the Kossoy Sisters.