The complex Young's modulus, E^*(ω) and the complex strain-optical coefficient, O^*(ω), of polystyrene(PS)/poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) blends were simultaneously measured and studied in the glass-to-rubber transition zone. The relaxation behaviors in E^*(ω) and O^*(ω) of three blends with different composition, were compared with each other in relation to the relaxation behaviors of constituents, PS and PPO. It was clarified that the PS/PPO blends seemed almost unique in the viscoelastic α-relaxation dispersion without respect to the compositions, which implied that the dissimilar constituents relaxed cooperatively. But the optical relaxation spectra, O^*(ω), of the blends showed distinct difference in quality. The signs of O^*(ω) of PS and PPO were opposite in the measured frequency range and blending of PS and PPO gave the reduction of birefringence coming from compensation of the birefringence of constituents. By increasing the incorporation of PPO in the blends, compensation of the opposite-signed birefringence of constituent polymers results in successive changes in sign of the O^*(ω) of the blends. Hence, we can expect that the composition of birefringence free blend may be in the 65~80% of PS fraction in PS/PPO blend. The properties of the opposite-signed birefringence also made us possible to make a quantitative evaluation of the contribution of each constituent to the relaxation of the blends by simple additivity rule. Compatibility of the blends was discussed in light of fractional contribution parameter.