On the late night host's show Wednesday, O'Brien and actor William Shatner poked fun at the 19-year-old father of Sarah Palin's grandson, with Shatner giving a dramatic reading of posts from Johnston's Twitter account - except Johnston claims he doesn't have an account on the site.
"Levi doesn't have a Twitter," his lawyer, Rex Butler, told E! News. "We thought it was common knowledge by now that there was an imposter Twittering in his name. Obviously, some outlets have chosen to run this anyway. A simple phone call before would have taken care of it."
On Thursday night's show, O'Brien apologized to Johnston for misrepresenting him.
"We'd been following Levi on Twitter and felt his gift for the written word needed to be shared with the world," he explained. "Today...I got some shocking news: Apparently the tweets that we read are counterfeit. They were written by a clever imposter posing as the witty Levi Johnston. And the real Levi is very angry with us."
"I would like to personally apologize to Levi Johnston and his lawyer and publicist for misrepresenting him in any way," he continued.
True to his word, O'Brien brought Shatner out again - but this time, to deliver quotes from Johnston that they said were verified.
"The first thing Sarah said to me at the hotel was, 'You gotta cut your hair.' I told her I didn't want to. I had a mullet at the time," Shatner read, getting laughter from the audience.
Johnston is also set to receive an award from an adult-themed Web site for baring it all in his upcoming Playgirl spread.
Adult Web site Fleshbot.com will be honoring the 19-year-old aspiring model and actor with the Crossover Award at its first annual Fleshbot awards, according to E! News. The nod recognizes Johnson's status as the "biggest pop culture star turned sex star."
The report says that Johnston is expected to be on hand to accept the award, which will be handed to him by the female star of the "Obama Girl" video.