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. He was joined by Judge
Cynthia Holcomb Hall in a contested 2–1 decision (judge
Marsha Berzon dissented). 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.
Consuelo María Callahan joined him in the majority, while
Sidney Runyan Thomas dissented.
Peruta was later overturned
en banc. 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. After that decision was found to be in error by the
Supreme Court of the United States and remanded, O'Scannlain, again, found the plaintiff had standing to sue. 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. The State of Hawaii requested a rehearing en banc and the Ninth Circuit held the hearing on September 24, 2020. O'Scannlain participated in the en banc hearing. On February 10, 2020, O'Scannlain wrote an opinion respecting the denial of en banc hearing in
Edmo v. Corizon. The original panel had ruled that a prisoner with
gender dysphoria had 8th amendment rights to sex reassignment surgery. 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. In the opinion, he mentioned that the 9th circuit is the first circuit to rule that denying gender reassignment surgery violates the 8th amendment, and that the 9th circuit is in conflict with other circuits' rulings, causing a circuit split. In September 2020, O'Scannlain wrote for the majority when it found that a
robocall defendant could not force the plaintiff into arbitration based on a customer agreement the plaintiff had signed with a separate company several years before it had been acquired by the holding company that now also owned the defendant. O'Scannlain also concurred separately to argue that even if the
Federal Arbitration Act had preempted state law limitation on
absurdity in contracts, it still did not require arbitration of claims wholly unrelated to the original contract. ==See also==