Diarmuid O'Scannlain holds a Juris Doctor from Harvard Law School and a Master of Laws from the University of Virginia School of Law. He began his career as a tax attorney for the Standard Oil Company of New Jersey and New York City, followed by private practice in Oregon. O'Scannlain has held various public sector roles, including Deputy State Attorney General and Director of the Oregon Department of Environmental Quality. He transitioned to federal judicial service, being nominated by President Ronald Reagan to the U.S. Court of Appeals for the Ninth Circuit. O'Scannlain has been influential in shaping judicial opinions on significant constitutional issues, including Second Amendment rights and gender dysphoria.

Diarmuid O'Scannlain's judicial philosophy reveals a blend of traditional conservatism with surprising judicial pragmatism. His support for splitting the Ninth Circuit showcases a strategic approach to federal court management, while his Second Amendment rulings exhibit a robust defense of gun rights. O'Scannlain's decision in Edmo v. Corizon highlights an unexpected alignment with progressive civil rights, creating a complex ideological tapestry. His rulings on consumer protection and arbitration further illustrate an unpredictable alignment of interests, prompting deeper inquiry into his judicial motivations.