He wrote on worker's control in his book The Tradition of Worker's Control. He also wrote Resisting the Nation State: The Pacifist and Anarchist Tradition (1991).
Ostergaard is probably best known for his two books on the Gandhian movement of the 1960s and 1970s in India. The Gentle Anarchists (co-authored with Melville Currell, 1971) and Nonviolent Revolution in India (1985). He wrote articles in the magazine Anarchy and Freedom (also under the name Gaston Gerard) and in Peace News. He had close contacts to the War Resisters' International.