Orange Eu²⁺-doped phosphors are essential for light-emitting diodes for cornering lights to prevent fatal road accidents at night, but such phosphors require features of high thermal, chemical stability and facile synthesis. This study reports a series of yellow-orange-red emitting SrAl₂Si₃ON₆:Eu²⁺ oxynitride phosphors, derived from the SrAlSi₄N₇ nitride iso-structure by replacing Si⁴⁺−N³⁻ with Al³⁺−O²⁻. The introduction of a certain amount of oxygen enabled the facile synthesis under atmospheric pressure using the air-stable raw materials SrCO₃, Eu₂O₃, AlN and Si₃N₄. SrAl₂Si₃ON₆ has a smaller band gap and lower structure rigidity than SrAlSi₄N₇ (5.19 eV vs 5.50 eV, Debye temperature 719 K vs 760 K), but exhibits higher thermal stability with 100 % of room temperature intensity remaining at 150 °C compared to 85 % for SrAlSi₄N₇. Electron paramagnetic resonance, thermoluminescence and density functional theory revealed that the oxygen vacancy electron traps compensated the thermal loss. Additionally, no decrease in emission intensity was found after either being heated at 500 °C for 2 hours or being immersed in water for 20 days, implying both of the thermal and chemical stability of SrAl₂Si₃ON₆:Eu²⁺ phosphors. The strategy of oxynitride-introduction from nitride promotes the development of low-cost thermally and chemically stable luminescent materials.