"Johanni van Oostrum verleiht der Agathe mit formidabler Phrasierungskunst einen Glanz, der insbesondere den hauchzarten Pianissimo Tönen innewohnt."

Opernwelt 07/22, "Fahrstuhl zum Fagott", Jürgen Otten


"Die in Sprüngen anhebende, weit atmende Kavatine intoniert ...die Südafrikanerin Johanni van Oostrum...so rein und schimmernd, dass der sie umgarnende Rote gleichsam ins Herz getroffen von der Couch rutscht."

FAZ, https://www.faz.net/aktuell/feuilleton/buehne-und-konzert/kirill-serebrennikows-freischuetz-in-amsterdam-18084068-p2.html, 07.06.2022


"Van Oostrums Marschallin war souverän, klar und kraftvoll."

Stefan Ender, "Trotz widriger Umstände gelungener Rosenkavalier", in: DerStandard, 8.4.22


 "Hermann Levi was a defender of Mozart’s operas. As was the custom in his day, works were performed in German in an arrangement he had made. Fiordiligi’s aria (from Così fan tutte) is challenging for all sopranos and the German text did not facilitate matters. South-African Johanni van Oostrum was glorious casting for this aria, she is a celebrated Elsa (Lohengrin) and has powerful high notes, plenty of colour and great care for the Mozartian line." 

Antoine Leboyer,"Krill petreko plays tribute to Hermann Levi", in: Seen and heard International, 24.07.2021


"Johanni van Oostrum sings Agathe’s glorious solos with ample tone and broad phrasing" 
 "The Freischütz project review", in: The Gramophone, April 2021 


(…) astonishing central performances from (...) Johanni van Oostrum as Agathe, truly outstanding (...) Van Oostrum sounded glorious in Leise, Leise and floated exquisite pianissimos in Und ob Die Wolke: this is an exceptionally beautiful voice. 
Tim Ashley “Der Freischütz review”, in: The Guardian, 05.11.2019
"Johanni van Oostrum’s superbly sung Salome... came very close to satisfying the composer’s almost impossible criteria. From her early “Elfenmusik” phrasing on “Wie gut ist’s, in den Mond zu sehn” with a peerless top A-flat, and the gentle pianissimo downward chromatic on “Elfenbein” followed by a delicious diminuendo A-flat on “Mond,” it was clear van Oostrum is an immensely accomplished singer with a rock-solid technique and rare gift for immaculately articulated word coloring. The voice is more on the lyric end of the scale but has plenty of meat and metal when required. The enormous vocal challenges of the part seemed effortless  – even the impossibly low G-flats such as “eine” before “Gruft” and the final “Todes,” which are usually just growled or grunted, were actually sung with an impressive chest voice. "
Jonathan Sutherland: „Oper Graz, Review: Salome”, in: Operawire.com, 07.12.2018