Polyisoprene (PI) and hydrogenated medium-vinyl polybutadiene (hPB), a polydiene-polyolefin pair, are expected to show limited compatibility (with a high interaction energy density, X, proportional to the Flory interaction parameter, chi). The regular mixing model suggests that styrene (S) units can boost the interblock compatibility when incorporated in small amounts into the hPB chain via random copolymerization. The mixing thermodynamics in symmetric polydiene-polyolefin "block-random" copolymers composed of PI and a random copolymer of styrene and hydrogenated medium-vinyl butadiene (hSBR) were investigated, through measurements of the order-disorder transition temperature. Block and block-random copolymers were prepared by anionic polymerization, followed by selective saturation of the butadiene units. More than a 2-fold decrease in X was achieved on incorporating similar to 20 wt % S in the random block, with a further decrease observed at similar to 30 wt % S incorporation, indicating a strong enhancement in compatibility. At higher styrene contents, X varied parabolically as qualitatively predicted by the regular mixing model, but better quantitative agreement was obtained with the copolymer equation model.