Refers to a theater specifically built for opera performances. Often used in proper names like パリ・オペラ座 (Paris Opera House).
パリのオペラ座でバレエを観た。
I saw a ballet at the Paris Opera House.
Compound of オペラ (opera) and 座 (seat/theater), a common suffix for traditional Japanese theaters (e.g., 歌舞伎座).