 After writing for several comedy shows in Los Angeles, he joined the writing staff of "Saturday Night Live".O'Brien was a writer and producer for "The Simpsons" for two seasons until he was commissioned by NBC to take over David Letterman's position as host of "Late Night" in 1993.A virtual unknown to the public, O'Brien's initial "Late Night" tenure received unfavorable reviews and remained on a multiweek renewal cycle during its early years.