Editorialist
Part of speech: noun
Definition: One who write opinion pieces, especially for a newspaper.
Columnist
Part of speech: noun
Definition: A regular writer of a column, such as in a magazine or newspaper
Example sentence: The great 'New York Times' columnist Dave Anderson famously slept one year in a child's race-car bed. There he was, Pulitzer Prize and all, snoring as his feet dangled over the rear tires of Lightning McQueen.