Oxygen (O₂) levels provide an important parameter in the early-stage diagnosis of cancers and other diseases. Ratiometric optical O₂ sensing has attracted much interest due to its better calibration of oxygen levels at two wavelengths. Oxygen sensing nanoprobes are generally designed by the incorporation of a reference dye and phosphorescent indicator into various matrixes. Silica nanomaterials with intriguing characteristic properties are highly preferable in hypoxia sensing. However, physical entrapment of dyes in silica suffers from leaching and moderate sensitivity. Reported herein is a covalently encapsulated dual-dye silica nanoprobe for ratiometric hypoxia detection and imaging. The nanoprobe prepared in a one-pot approach has a core–shell nanostructure where silanized indicators and reference dyes are covalently grafted within the silica core, while poly(ethylene glycol) chains form an outer shell. To our delight, through the covelent bonding of the dyes for efficient energy transfer between the reference dye and the indicator, high phosphorescence brightness, long lifetime and excellent stability of the nanoprobe were achieved. More importantly, the designed nanoprobe can respond to different oxygen levels in both solution and living cells with impressive sensitivity and stability.