Rubuta ci gaba don wannan sakin layi - A ranar 1 ga Yuli, 2018, an yi musayar O'Reilly zuwa St. Louis Blues a musayar Tage Thompson, Vladimír Sobotka, Patrik Berglund, a 2019
zaɓaɓɓen zagaye na farko da zaɓaɓɓen zagaye na biyu na 2021. Kodayake O'Reilly ya sami babban kakar wasa tare da Blues, ƙungiyar ta faɗi zuwa matsayi na ƙarshe a duk faɗin gasar ta Janairu 2, wanda ya haifar da sauya mai horarwa da mai tsaron gida. Yayin da Blues suka sake farfadowa, O'Reilly ya ji daɗin shekararsa a shekararsa ta farko a St. Louis, inda ya sami nasarori masu yawa a cikin taimakawa da maki kuma ya daidaita aikinsa a raga. An gayyace shi zuwa na biyu na wasan NHL All-Star Game, inda ya yi rajista da maki bakwai ta hanyar wasanni biyu kuma ya kasance a wasan karshe na MVP na All-Star Game, wanda ya rasa kyautar ga Sidney Crosby.