O'Brien moved to Los Angeles after graduation to join the writing staff of HBO's Not Necessarily the News. He was also a writer on the short-lived The Wilton North Report. He spent two years with that show and performed regularly with improvisational groups, including The Groundlings. In January 1988, Saturday Night Live's executive producer, Lorne Michaels, hired O'Brien as a writer. During his three years on Saturday Night Live (SNL), he wrote such recurring sketches as "Mr. Short-Term memory" and "The Girl Watchers"; the latter was first performed by Tom Hanks and Jon Lovitz. O'Brien also co-wrote the sketch "Nude Beach" with Robert Smigel, in which the word "penis" was said or sung at least 42 times. While on a writers' strike from Saturday Night Live following the 1987-88 season, O'Brien put on an improvisational comedy revue in Chicago with fellow SNL writers Bob Odenkirk and Robert Smigel called Happy Happy Good Show. While living in Chicago, O'Brien briefly roomed with Jeff Garlin. In 1989, O'Brien and his fellow SNL writers received an Emmy Award for Outstanding Writing in a Comedy or Variety Series.  O'Brien, like many SNL writers, occasionally appeared as an extra in sketches; his most notable appearance was as a doorman in a sketch in which Tom Hanks was inducted into the SNL "Five-Timers Club" for hosting his fifth episode. O'Brien returned to host the show in 2001 during its 26th season. O'Brien and Robert Smigel wrote the television pilot for Lookwell starring Adam West, which aired on NBC in 1991. The pilot never went to series, but it became a cult hit. It was later screened at The Other Network, a festival of unaired TV pilots produced by Un-Cabaret; it featured an extended interview with O'Brien and was rerun in 2002 on the Trio network.  Things changed for O'Brien in 1991, when in quick succession, an engagement fell through; Lookwell was not picked up; and, burned out, he quit Saturday Night Live. "I told Lorne Michaels I couldn't come back to work and I just needed to do something else," O'Brien recalled. "I had no plan whatsoever. I was literally in this big transition phase in my life where I decided, I'll just walk around New York City, and an idea will come to me." Mike Reiss and Al Jean, then dual showrunners of The Simpsons, called O'Brien and offered him a job. The series was notorious in the writing community at the time; O'Brien recalls "everyone wanted to be on that show, but they never hired." O'Brien was one of the first hires after the show's original crew. With the help of old Groundlings friend Lisa Kudrow, O'Brien purchased an apartment in Beverly Hills. He and Kudrow became involved as well, and Kudrow believed he should begin performing rather than writing. O'Brien disagreed, feeling that Kudrow was being overly flattering and that asserting he was happy as a writer. In his speech given at Class Day at Harvard in 2000, O'Brien credited The Simpsons with saving him, a reference to the career slump he was experiencing prior to his being hired for the show.

Answer this question "what did he do before this" by extracting the answer from the text above.
He was also a writer on the short-lived The Wilton North Report. He spent two years with that show and performed regularly with improvisational groups, including The Groundlings.