In 1932, as a result of a family legacy, he moved to Sydney, where he established the Oldham Publishing Company which produced lino-cut posters and calendars.
In the late 1930s the couple joined the Communist Party and embraced its programmes – especially the Workers’ Art Guild.
They moved to Sydney, where he joined the architectural firm of Stephenson and Turner and, in 1939, was given the task of designing the Australian Pavilion for the 1939 New York World's Fair.
Among his many achievements were the landscaping of the Narrows Interchange, Parliament House, Sir Charles Gairdner Hospital, Serpentine and Wellington Dams, Western Australian Institute of Technology, Crestwood estate and developing a master plan for Kings Park and the Swan River foreshores.
Internationally renowned as a landscape architect, Oldham was admitted as a Member of the Order of Australia (AM)in 1990.