The O'Neill Building is a landmarked former department store, located at 655-671 Sixth Avenue between West 20th and 21st Streets in the Flatiron District neighborhood of Manhattan, New York City. The building was originally Hugh O'Neill's Dry Goods Store, and was designed by Mortimer C. Merritt in the neo-Grec style. It was built to four stories in two stages between 1887 and 1890, to allow the existing O'Neill store to continue operating during construction, with the addition of a fifth floor in 1895, created by raising the pediment. The gilded corner domes of this cast-iron-fronted building were restored c.2000.