He is best known for his work on the Ant chair, produced by Fritz Hansen in 1952, which has become an icon of Danish design. The Ant chair is characterized by its organic form and laminated wood construction, making it both elegant and comfortable.
Ostergaard also worked on other furniture, such as tables and chairs, as well as smaller design objects, such as clocks and lamps.
His work has been exhibited in several museums, including New York's Museum of Modern Art and London's Victoria and Albert Museum. Today, the Ant chair is still produced by Fritz Hansen and is considered one of the greatest achievements of Danish design.