That's funny, same thing with me first time I saw Holy Grail. It was in a church basement or library basement, someplace like that, a free show on a cheap blurry screen, I sat through it without paying attention.
Raising Arizona is great, too. What the hell happened to Nicolas Cage? He's still hilarious, I guess, but no longer intentionally so.
Before you watch O Brother, I'd recommend you see Sullivan's Travels first. It'll make for a more rewarding experience.