In September 1960, O'Scannlain attended the founding conference of Young Americans for Freedom, held at William F. Buckley Jr.'s estate in Sharon, Connecticut.

On August 11, 1986, President Ronald Reagan nominated O'Scannlain to a seat on the United States Court of Appeals for the Ninth Circuit vacated by Judge Robert Boochever.

That nomination was withdrawn after controversial newspaper writings by Bounds as a student at Stanford University on the subject of the civil rights of minorities and women were discovered.

[11] In a controversial March 2010 case, O'Scannlain joined the majority opinion that Seattle police officers did not employ excessive force when they tasered a pregnant woman.

On February 13, 2014, O'Scannlain wrote the majority opinion in the case of Peruta v. San Diego, and issued a ruling that stated California's may-issue concealed carry rules, as implemented by the County of San Diego, in combination with a ban on open carry in most areas of the state, violate the Second Amendment, because they together deny law-abiding citizens the right to bear arms in public for the lawful purpose of self-defense.

In Spokeo, Inc. v. Robins, O'Scannlain found that under the Fair Credit Reporting Act a plaintiff had standing to sue an allegedly inaccurate website.

[16][17] On July 24, 2018, O'Scannlain wrote the majority opinion in the case of Young v. Hawaii, which said that the Second Amendment protects the right to open carry in public.

O'Scannlain explained that as a judge in senior status, he cannot vote on these petitions, but he can issue statements respecting the denial of en banc.