My take:
They are two of my favorite movies. One movie has "cool factor", the other is just hysterical. OS is just funnier throughout. Lebowski centered on a character that really wasn't that funny. He was just extremely cool in his obliviousness. I would bet that most guys that like these movies would love to be a Lebowski kind of person. Even though they are both losers, nobody wants to be a loser like Hank the Tank.
I think that John Goodman carried the comedy aspect of that film pretty much on his own. Dude had a couple of funny lines, like "Where's the money Lebowski? I don't know, let me go down and take another look" still makes me laugh when I think about it. But Goodman was spot-on funny. There's no doubt Lebowski was much better written movie though. The characters and plot were much more defined and memorable. But at the end of the day, I think Old School was funnier.
Other crappy movies like OS that were extremely funny are Hot Tub Time Machine and Hall Pass.

Old School is
funnier
.
If I need to laugh, Old School will reliably make me laugh many times.
I'm talking belly laughs, uncontrollable, convulsing laughter.
The Big Lebowski was clever, and funny,
but it didn't make me piss myself laughing.
Some of you have said that TBL is "art" and "the greatest movie of all time." That doesn't answer the question, which movie is funnier (you retards).
I only watched TBL once. Ppl are saying you have to watch it more than once to "get it." So I'll watch it again. Funniest thing I remember is,
"You want a toe? I can get you a toe."
UPDATE: I just rewatched TBL, for the second time. It was goodmaybe better than Old School, but not funnier. John Goodman was hilarious, as was Bill Murray's hair.
P.S. I've read many comments saying that if you don't think TBL is funnier than Old School, then you're not smart enough to "get it." That you need an "intellect" to appreciate why TBL is hilarious. I see many of these comments, but without any support. I can explain exactly why I think it's funny when Walter constantly snaps at Donny, "Shut the beep up, Donny!" and "Life does not stop and start at your convenience you miserable piece o' beep That's hilarious because Donny doesn't seem to provoke that unreasonable reaction.
However, most of you fans are quoting lines that I don't find funny at all, e.g., "im the dude man so thats what you call me, y know."
How the beep is that funny? I seriously doubt you have a "bigger intellect" than I have, and that's why you see humor in it, you miserable piece of beep

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Kingpinbut actually old school and the big lebowski are Unbelievably great. John Goodman and Jeff Bridges are fantastic.

I don't understand how this is even up for debate. Lebowski is great, Old School is dumb. The end. That's like comparing Rob Schnieder to Daniel Day-Lewis.

Big Lebowski by far!
Prostitute: What the beep are you doing?
Johnny: I'm gonna kill a bunch of people.

Well Old School is my favorite comedy in the vein of Super Bad and stuff but The Big Lebowski is my favorite movie ever. Ever ever? Ever.

Old School x 100000, but I've never been a big fan of the Big Lebowski. In my opinion people froth over it because of who made it.

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WTF!!!! How could you compare The Big lebowski with Old school!!!!!! Lebowski was way too funny and cool

The Big Lebowski
.
Born when she kissed me, died when she left me, lived whilst she loved me

I loved the Big Cat, but he can't hold a candle against The Duderino!!
now this is acting: http://www.imdb.com/media/rm2458172160/tt1528718

"The Big Lebowski is quite possibly the most overrated movie in the history of cinema. I mean it has a few funny moments but come on!"
Agreed. Good movie but HIGHLY overrated. Especially by hipsters.