In 1930 O'Brien became a founding member of the Beach Erosion Board having been hired two years earlier by its predecessor organization. His interest in beach erosion led him to author a seven-volume study of the entire United States Pacific coastline, "A Report on Sand Movement and Beach Erosion Along the Pacific Coast of the United States", in which he both addressed every American inlet and harbor on the Pacific and documented the phenomenon of littoral drift. O'Brien continued to serve as a member of the Beach Erosion Board and beyond 1963, its successor organization, the Coastal Engineering Research Center until his retirement in 1978. In the late 1930s O'Brien did consulting work for Weyerhaeuser Company to improve the flow in their log ponds. He also provided consultation on the placement of caissons during the construction of the ill-fated Tacoma Narrows Bridge. O'Brien had also been on the consulting board of the Waterways Experiment Station.