Strain-attenuated spin frustration in double perovskite <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msub><mml:mi>Sr</mml:mi><mml:mn>2</mml:mn></mml:msub><mml:mi>Fe</mml:mi><mml:mi>Os</mml:mi><mml:msub><mml:mi mathvariant="normal">O</mml:mi><mml:mn>6</mml:mn></mml:msub></mml:mrow></mml:math>

Using density functional theory together with Monte Carlo simulations, we demonstrate that epitaxial strain, both compressive and tensile, attenuates the spin frustration of double perovskite ${\mathrm{Sr}}_{2}\mathrm{Fe}\mathrm{Os}{\mathrm{O}}_{6}$ to significantly enhance critical temperature 310 K, enabling room-temperature applications. We discover under tensile strain a tetragonal ($I4/m$)-to-monoclinic ($P{2}_{1}/n$) structural transition concomitant an antiferromagnetic-to-ferrimagnetic transition. Furthermore, indirect-to-direct band gap is observed valence conduction states localized on different metal sublattices, opening route efficient electron-hole separation upon photoexcitation.