Moments after he took the oath, O'Brien was presented with the gavel by former Speaker Bob O'Donnell.
"I am truly overwhelmed. This is not something I thought about 24 hours ago. This is going to be one of the shortest speeches in Pennsylvania history. This is unexpected and I'm completely honored," O'Brien said with his son, Dennis, at his side.
He went on to pledge that he would move forward with a series of unspecified reforms.