Initially a resident in pathology, he soon became an assistant professor of pediatric surgery at Harvard and later moved to Boston's Floating Hospital for Children as surgeon-in-chief, where he was the first pediatric surgeon on the staff at the hospital and where he began his research in Hirschsprung's disease.

[1] His parents, Amanda and Carl Albert Swenson, were missionaries for the Community of Christ and relocated their family to Independence, Missouri, in 1917.

[1] Both parents died when Orvar was a teenager, and he and his brother Alvin lived in a boarding house where they started a business, Woodcraft, which sold fire-by-friction sets, bows and arrows, and field hockey sticks.

[1][2] The same year, Orvar and Alvin were admitted to Harvard Medical School, where they successfully petitioned the dean to be placed in the same class so that they could share textbooks in order to save money.

He described the hallmark clinical and radiological markers of Hirschsprung's disease in newborns and showed that the only way to make a definitive diagnosis was to perform a full-thickness rectal biopsy.

[3] Over his career, he traveled to India, Australia, South America, Europe and Canada to demonstrate his procedure.

He was among the first to advocate performing a pelvic osteotomy when treating bladder exstrophy and for partial nephrectomy in bilateral Wilms' tumors.