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In this paper, we investigate by ab initio DFT how the O:H ratio influences formation and lattice energy, metastability, optical properties of Y La anion-disordered $R{\mathrm{O}}_{\mathrm{x}}{\mathrm{H}}_{3\ensuremath{-}2\mathrm{x}}$ oxyhydrides. To achieve this, a set special quasirandom structures (SQS) is introduced to model anion-disorder along whole ${R\mathrm{H}}_{3}\ensuremath{-}{R}_{2}{\mathrm{O}}_{3}$ composition line. A comparison with an extensive anion-ordered polymorphs same shows comparable energy phase, which, in particular, H-rich interval showed lowest relative energy. turn, metastability phase depends on cation size (Y versus La), which determines maximum H content above ${\mathrm{CaF}}_{2}$-type structure itself becomes unstable. overcome accuracy limitations classical DFT, modified Becke-Johnson (mBJ) scheme employed study electronic properties. We show that major differences occur between O-rich R oxyhydrides, as octahedral ${\mathrm{H}}^{\ensuremath{-}}$ present for $x&lt;1$ form states at top valence band, reduce band gap dominate transitions lower energies, thus increasing refractive index material VIS-nIR spectral range. Comparing results experimental data photochromic oxyhydride films reinforces hypothesis ($x&lt;1$), while it hints towards some degree anion ordering ones ($x&gt;1$). Our paper exemplifies strategy calculate electronic/optical wide range materials occupational disorder.