The isotope effect in superconductors can be influenced by electronic structure features such as flat bands and Van Hove singularities, providing a pathway for tuning superconducting properties. Using first-principles calculations within the framework of conventional electron-phonon coupling in the harmonic approximation, we predict superconductivity in the oxygen superhydrides ${\mathrm{H}}_3\mathrm{O}, {\mathrm{D}}_3\mathrm{O}$, and ${\mathrm{T}}_3\mathrm{O}$ for Coulomb pseudopotential ${\mu }^{*}$ values ranging from 0.1 to 0.13. Notably, the compressed ${\mathrm{H}}_3\mathrm{O}$ exhibits an extraordinary near-room-temperature superconducting transition at 301.5 K, while the corresponding ${\mathrm{D}}_3\mathrm{O}$ and ${\mathrm{T}}_3\mathrm{O}$ show superconducting transitions at 247 and 218.8 K, respectively, in accordance with the expected isotope effect. To characterize the flat band near the Fermi level, which is closely related to the superconductivity in these systems, we define the flat-band depth $d$ as its energy distance below the Fermi level. Our calculations reveal a systematic decrease in $d$ with increasing hydrogen isotope mass, while the height $h$ of the Van Hove singularity at the Fermi surface increases with increasing isotope mass. We uncover a striking linear relationship between the superconducting critical temperature and the product of $d$ and $h$ and further demonstrate the universal existence of this linear relationship across the chalcogen superhydrides. By separating the pure phonon-mass effect from the isotope-induced electronic-structure evolution, we show that the latter provides a significant contribution to $T_c$, quantitatively captured by the product $d\times h$. These findings provide interesting insights into the interplay between flat-band physics and the isotope effect in superconductors, offering instruction for the design of novel high-temperature superconductors.

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• Isotope effect
• Superconductivity
• High-temperature superconductors
• First-principles calculations