Metal hydride oxides are an emerging field in solid-state research. While some lanthanide hydride oxides (LnHO) were known, YHO has only been found in thin films so far. Yttrium hydride oxide, YHO, can be synthesized as bulk samples by a reaction of Y2O3 with hydrides (YH3, CaH2), by a reaction of YH3 with CaO, or by a metathesis of YOF with LiH or NaH. X-ray and neutron powder diffraction reveal an anti-LiMgN type structure for YHO (Pnma, a = 7.5367(3) angstrom, b = 3.7578(2) angstrom, and c = 5.3249(3) angstrom) and YDO (Pnma, a = 7.5309(3) angstrom, b = 3.75349(13) angstrom, and c = 5.3192(2) angstrom); in other words, a distorted fluorite type with ordered hydride and oxide anions was observed. Bond lengths (average 2.267 angstrom (Y-O), 2.352 angstrom (Y-H), 2.363 angstrom (Y-D), >2.4 angstrom (H-H and D-D), >2.6 angstrom (H-O and D-O), and >2.8 angstrom (O-O)) and quantum-mechanical calculations on density functional theory level (band gap 2.8 eV) suggest yttrium hydride oxide to be a semiconductor and to have considerable ionic bonding character. Nonetheless, YHO exhibits a surprising stability in air. An in situ X-ray diffraction experiment shows that decomposition of YHO to Y2O3 starts at only above 500 K and is still not complete after 14 h of heating to a final temperature of 1000 K. YHO hydrolyzes in water very slowly. The inertness of YHO in air is very beneficial for its potential use as a functional material.