In World War II, he served in the Army Air Forces and was responsible for water safety at one of the D-Day invasion camps.
[4][5] Oswald remained in the Army in Europe after the war, working in a hospital where he cared for patients suffering from waterborne ailments and met his lifelong wife, an American nurse.
[4] Oswald studied the role of algae in wastewater treatment as a student, winning recognition from the American Society of Civil Engineers for his early work.
[5] He continued investigating the use of microalgae in sanitation throughout his career, developing strategies for combining engineered algae ponds to obtain specific water treatment goals.
[5][10][11] An archive of Oswald's work is maintained by former student and long time collaborator Tryg Lundquist, now a professor of Environmental Engineering at Cal Poly, San Luis Obispo.