Soprano Johanni van Oostrum was born in South Africa and studied at the University of Pretoria. She received her vocal training with Dr Mimi Coertse and was a member of the Black Tie Opera Studio. Being a sought-after interpreter of the great soprano roles of Strauss, Weber, Janáček and Wagner, among others, Johanni van Oostrum works in this repertoire with conductors such as Sir Simon Rattle, Alan Gilbert, Ivor Bolton, Yannick Nézet-Séguin, Adam Fischer, Antonello Manacorda, Oksana Lyniv or Laurence Equilbey, as well as with directors such as Harry Kupfer, Barrie Kosky, Tobias Kratzer or Vera Nemirova. 

One of her leading roles is the Feldmarschallin (Der Rosenkavalier), which she has sung under Sir Simon Rattle at the Nederlandse Opera in Amsterdam, at the Royal Swedish Opera in Stockholm under Alan Gilbert, at the Komische Oper Berlin or at the Bolshoi Theatre in Moscow. She celebrated a triumphant success in 2022 with her debut at the Vienna State Opera - as a stand-in in this very role in Otto Schenk's legendary production of Der Rosenkavalier
After her debut at the Bavarian State Opera as Contessa, she also won over audiences and critics alike there with a brilliant stand-in as Elsa (Lohengrin). Johanni van Oostrum sang the title role in Richard Strauss' Salome at the Graz Opera and Marietta in Korngold's Die tote Stadt at the Opéra de Limoges - both role debuts. 
In 20/21, despite the pandemic, she made her debuts as Leonore in a concert performance of Beethoven's Fidelio, conducted by Adam Fischer, and in the title role of Leos Janacek's Jenůfa at the Konzert Theater Bern. She was Agathe in "The Freischütz Project" by Laurence Equilbey with the Insula Orchestra, which was performed live in Aix-en-Provence, the BOZAR in Brussels, the Theater an der Wien, the Théâtre des Champs-Élysèes in Paris and the Barbican Centre in London and has been released on CD and DVD by Warner Classics and Erato respectively. Planned - and prevented by the pandemic - were her debuts at the Liceu Theatre in Barcelona as Elisabeth in Wagner's Tannhäuser and a new production of Le nozze di Figaro at the Canadian Opera Company. In May 2022 she was acclaimed as Agathe in Kirill Serebrennikov's acclaimed new production of Freischütz at the Nederlandse Opera in Amsterdam. 
Johanni van Oostrum opens the 22/23 season with her debut as Elisabeth in the new production of Tannhäuser at the Opéra national de Lyon, with Daniele Rustioni conducting for the first time in a Wagner opera. She subsequently appears as Chrysothemis in Hamburg, as Eva in Die Meistersinger at the Tokyo Spring Festival under Marek Janowski, in the title role in a new production of Dvorak's Rusalka at the Amsterdam Opera, and finally again as Agathe, this time at the Dresden Semperoper. 
On the concert stage, the versatile singer can be heard performing Beethoven's 9th Symphony, Four Last Songs and other orchestral songs by Richard Strauss, Beethoven's concert aria Ah! Perfido, the German Requiem by Johannes Brahms, Mendelssohn's Elijah, Zemlinsky's Lyric Symphony and Verdi's Messa di Requiem. A special concern of hers is the performance of songs by South African composers.