O'Brien was approved to run against Liberal MP Scott Simms, just one day after the party rejected Ches Crosbie as a Conservative candidate in Avalon.
While Crosbie was reportedly rejected over a joke he told at a fundraiser that poked fun at Prime Minister Stephen Harper, O'Brien has been accepted despite harsh comments he made about Harper in the past.
O'Brien previously said that Harper had "no integrity" on VOCM's Backtalk several years ago.
"Certainly, we do want to work with Canada and we want to work with the federal government," he said. "The fact the man made promises and he broke promises."
Party insiders had told CBC News those comments could mean O'Brien's rejection as a candidate.
Now that he is the official candidate for the federal election, O'Brien said he will resign as the MHA for Gander.
The seat will remain vacant until the provincial election in November.