In the film, Oh Brother Where Art Thou?, there are many themes that can be explored. Here are a few that stand out to me.
There are many scenarios in the film that bring to mind the theme of gullibility in human nature. Whole groups of people are tricked into supporting a particular political candidate due only to his support of a popular music group. The main characters begin their adventure because they whole heartedly believe Everett’s lie that he knows where they can find a treasure. Then, they proceed be bamboozled by everyone who has the upper hand over them from beautiful singing women, to a smooth talking, one eyed bible salesman. Since the film is a comedic farce the point could be argued that the film itself is a warning not to be gullible, and not to get sucked into any story told by a fancy talker in a suit.
Another theme of the movie involves poking fun at and revealing the ridiculousness of cultural stereotypes revolving around the deep south. The characters are involved in numerous elaborate gags that highlight the ridiculousness of such beliefs as white supremacy, the superiority of big city folk, and the infallibility of ministers and politicians.
The theme of role reversal with the criminals as the heroes and lawmen as the villains runs through the whole film. The sheriff and police are time after time compared to devils and revealed as crooks, while the criminals are portrayed as good hearted, loyal, and the overall force of good.
Another pervading theme in Oh Brother Where Art Thou? is the importance of loyalty to friends and family. Everette risks his life and the lives of his companions to get back to his family and to the role of “pater familias” a powerful term from ancient Rome that means,
The head of a Roman family (who) held legal privilege over the property of the familia, and varying levels of authority over his dependents: these included his wife and children, certain other relatives through blood or adoption, clients, freedmen and slaves. (eNotes/Wikipedia page on Pater Familias)
The characters do make mistakes that involve betraying their friends, but they always redeem themselves in a poignant scene of penitence that leads the audience to see the importance of being true to your ‘boon companions.’