In a series of video messages promoting Monday night's appearance on Conan, Ferrell teased that he was only agreeing to guest on the TBS late night series in order to restore O'Brien's formerly clean-shaven look.
The presenter famously grew a beard after being sacked as host of The Tonight Show in January 2010 and kept the facial hair when his new series premiered on TBS last fall.
After being introduced by O'Brien, Ferrell took to the stage on Monday night's telecast and made light of Osama Bin Laden's recent death by insisting that the world is fixated on the talkshow host's whiskers.
"There is nothing else going on in this country other than the fervour to shave that beard," Ferrell joked. "I turned on the television last night to see if people were talking about it. There were people celebrating in front of the White House. There were people celebrating in Times Square chanting 'USA! Shave the beard!'"
O'Brien quickly retorted: "I'm not sure you heard them correctly."
After producing shaving cream and a razor, Ferrell proceeded to cut off much of O'Brien's facial hair, adding: "I've only shaved dogs before!"
Upon completion of his barber work, the Office guest star feigned horror as he glimpsed O'Brien's half-shaved face.
"I made a terrible mistake," he shouted.
Ferrell has made frequent appearances on each of O'Brien's three late night shows since joining Saturday Night Live in 1995. He famously served as the final guest on The Tonight Show with Conan O'Brien last January.
Watch Will Ferrell shave Conan O'Brien's beard below: