In this work, we present a rapid, selective and highly sensitive sensor for the detection of oxygen based on ratiometric fluorescent luminescent carbon dots (CDs) grafted CaSiO₃:Eu³⁺. Highly monodispersed porous silica microspheres are obtained with particle sizes in the range between 120 and 180 nm, providing a key precursor to prepare the CaSiO₃:Eu³⁺ phosphor. The fluorescence of CDs grafted CaSiO₃:Eu³⁺ could be quenched by oxygen and the red emission of Eu³⁺ (613 nm and 703 nm) and the blue emission of the CDs (454 nm) were effectively reduced as the concentration of oxygen increased. The CaSiO₃:Eu³⁺ phosphor shows little effect on oxygen, while CDs grafted CaSiO₃:Eu³⁺ shows good oxygen sensing properties, the response time is determined to be about 4–7 s and the recovery time is 18–29 s. The obtained CDs grafted CaSiO₃:Eu³⁺ sample appears to be a promising sensing material for environmental detection applications and would also find applications in catalysis, electrodes, or related fields.