<p>Rational design of birefringent crystals with high birefringence remains a critical challenge. Herein we present two oxalate crystals of (C₆N₂H₁₁)(HC₂O₄) (<b>1</b>) and (C₄N₂H₄)(H₂C₂O₄) (<b>2</b>) (H₂C₂O₄ = oxalic acid, C₆N₂H₁₁ = 2-ethyl-4-methylimidazolium cation and C₄N₂H₄ = pyrazine). Remarkably, crystal <b>2</b> exhibits an unprecedentedly large birefringence of 0.422 at 550 nm, which surpasses all commercial birefringent crystals. The formation of the directional hydrogen bonds between oxalate and planar pyrazine constituents facilitates the adoption of a planar configuration by oxalic units possessing large polarizability anisotropy (Δ<i>α</i> = 21.72). Whereas the distorted configuration of oxalate groups with a relatively small Δ<i>α</i> of 13.95 induced by the noncoplanar arrangement of the imidazole planes of C₆N₂H₁₁ leads to moderate birefringence (0.144@550 nm) for <b>1</b>. Computational analyses reveal that the birefringent superiority of <b>2</b> originates from the synergistic effect of the π-conjugated oxalate and pyrazine units in a parallel arrangement directed by hydrogen bonds. This work breaks the record of birefringence in oxalates. It also develops a powerful hydrogen bond-directed strategy to modulate the configuration of oxalate groups, enabling its use as a tunable anisotropic structural unit for constructing birefringent crystals.</p>