Configurational-bias Monte Carlo simulations in the Gibbs ensemble are used to study the thermodynamic and structural properties associated with the miscibility of binary olefin oligomer mixtures representing poly(ethylene-alt-propylene), polypropylene, and head-to-head polypropylene. Single-component simulations are performed to compute the cohesive energy densities, Π_CED, of different oligomers that are often utilized in estimating the miscibilities of compounds in the liquid phase but are not measurable for high-boiling compounds, such as polymers. Extrapolating simulation data for C5 to C36 oligomers allows for determination of the infinite-chain-length Π_CED values of three polyolefins. The results agree remarkably well with values deduced from small-angle neutron scattering experiments on high-molecular-weight polymers. In addition, the Flory–Huggins χ parameters based on the free energy of mixing for pairs of olefins are calculated directly from simulations of binary mixtures. The binary propylene and head-to-head propylene oligomer blend is found to exhibit stabilized irregular mixing behavior, in agreement with its polymeric counterpart. This chain-length independence of the mixing behavior is interpreted via insights from structural analysis. Our results identify simulations of oligomeric systems as a promising route to predict and understand polymer blend phase behavior.