"I want to make sure I don't over-think it," O'Brien told a meeting of the Television Critics Association on Thursday. "Television is changing drastically. I want to make sure my show isn't too buttoned-up."
It's archaic to think the comedy that works on a post-midnight show like his current "Late Night" won't work an hour earlier on "Tonight," O'Brien said. Besides, he said, only so much change is possible.
But his "Tonight" won't be a continuation of "Late Night," O'Brien said, and inevitably will evolve as he does the job and settles into his new Los Angeles studio and life.
The show's pieces are being assembled, including deciding which writers and other staff members will come to Los Angeles, he said then couldn't resist cracking a joke.
"Everyone's been fired," O'Brien said.
He's set to replace Jay Leno as "Tonight" host in June, with Leno moving to a new daily prime-time show for NBC. Former "Saturday Night Live" cast member Jimmy Fallon will take over "Late Night" in March.
"I'm just going to run my race and do the best show I can. I don't think I can touch David Letterman 's legacy in television," he said.
O'Brien said he doesn't want to make his "Late Night" farewell a "big salute to me," since he's moving on to a dream job and not being rocketed to Mars.
He had kind words for his replacement, calling Fallon funny and likable and predicting that "he's going to do fantastic."
In an earlier session Thursday with TV critics, Fallon said it was gracious of his future competitor, CBS' "Late Late Show" host Craig Ferguson , to suggest that reviewers wait a month before assessing Fallon's performance.
"I'm up against a really classy guy. I actually sent him a gift basket today with a collection of Sean Connery movies," Fallon said. Both Connery and Ferguson are Scottish.
Fallon said he figures his real competition is the fact that people want to go to sleep after midnight, not watch TV, and his job is to keep them interested and awake.