Try the remake of “True Grit?” It’s also a Coen Brothers movie about odd characters on a journey. The tone is totally different; in O Brother, everything was a lot more slapstick-funny and it had an awesome soundtrack, so it’s almost like watching an old-timey musical. True Grit is more of a straight-up western, but with funny, clever dialogue.
“Two Mules for Sister Sara” also has a nice combination of adventure and humor. Clint Eastwood is… Clint Eastwood, and Shirley Maclaine is a potty-mouthed nun, and they go on an adventure together.
This is a different genre and the tone is a bit more laid-back, but the anime Samurai Champloo is actually a really good fit. It’s about three mismatched characters who go on a journey across 1800s Japan… sort of. There are tons of anachronisms, quirky, modern-day humor, and a great laid-back hip-hop soundtrack. The storytelling and character build are awesome, but it’s also a lot of fun.