Karl Aagaard Østvig (sometimes Germanized to Oestvig) (17 May 1889 – 21 July 1968) was a Norwegian operatic tenor, opera director, and voice teacher. The Norwegian Music History Archive states that he is the greatest dramatic tenor in Norwegian history, and that he was particularly admired for his portrayals in operas by Giacomo Puccini, Richard Strauss and above all Richard Wagner. His voice was described by soprano Lotte Lehmann as "a dazzling meteor". Although he toured internationally in concerts, operas, and recitals, his singing career was mainly centered in Austria and Germany from his debut in 1914 up until his retirement from the stage in 1933. From 1932 on he lived in Oslo where he was primarily active as an opera director and singing teacher. He made only a few recordings during his career, all for Polydor Records.
Born in Oslo, Østvig studied singing privately with Wilhelm Cappele Kloed in Germany, and at the Hochschule für Musik Köln with Fritz Steinbach and Franz Wüllner. He made his professional opera debut at the Staatsoper Stuttgart in 1914 where he was a resident artist for the next five years. While there he notably created the role of the Lay Brother/Giovanni de Salviati in the world premiere of Max von Schillings's Mona Lisa in 1915. He appeared as a guest artist at Theater Basel in 1917 and at the Bavarian State Opera in 1919.
From 1919-1926 Østvig was the leading tenor at the Vienna State Opera (VSO). With that company he performed the role of the Emperor in the world premiere of Richard Strauss's Die Frau ohne Schatten opposite Maria Jeritza as the Empress. He sang opposite Jeritza in many other operas at the VSO, including Bacchus to her Ariadne in Ariadne auf Naxos and Paul to her Marie in Die tote Stadt. His other roles in Vienna included Cavaradossi in Tosca, Don José in Carmen, Manrico in Il trovatore, Pedro in Tiefland, Pinkerton in Madama Butterfly, Siegmund in Die Walküre, Tamino in The Magic Flute, Walther in Die Meistersinger von Nürnberg, and the title roles in Lohengrin, Parsifal, Tannhäuser, and The Tales of Hoffmann. While working at the VSO he married soprano Maria Rajdl. Their daughter, Lillemari Østvig, had a career as a concert and opera soprano.