conduct that constitutes infringing activity," O'Malley wrote.
The unanimous Federal Circuit panel ordered further proceedings before Alsup to decide whether Google's actions were protected under fair use.
Programmers could still craft interoperable programs if the opinion stands, but lawyers will have to be more involved in signing off on what is permissible, said Eric Goldman, a professor at Santa Clara University School of Law.
"That's really expensive and lawyers are not going to give yes or no answers, and that's going to be stressful for everybody," Goldman said.
Google had argued that software should only be allowed to be patented, not copyrighted. However, O'Malley wrote that the Federal Circuit is bound to respect copyright protection for software, "until either the Supreme Court or Congress tells us otherwise."
Oracle General Counsel Dorian Daley called the decision a "win" for an industry "that relies on copyright protection to fuel innovation." Google said it set a "damaging precedent for computer science and software development" and was considering its options.
The case in the U.S. Court of Appeals for the Federal Circuit is Oracle America Inc vs. Google Inc, 13-1021.