By 1919, Kahn had assembled a estate on Long Island, and had Oheka Castle (from "Otto Hermann Kahn") built as its centerpiece. At , the 127-room structure was designed as the second-largest private residence in the United States (after George Vanderbilt's Biltmore Estate in Asheville, North Carolina) by Delano & Aldrich of New York City; its landscaping was designed by Olmsted Brothers, sons of Frederick Law Olmsted of Brookline, Massachusetts. The property featured a golf course, a working farm, a private airstrip, and numerous outbuildings.