A series of compounds based on peroxide-cured ethylene–propylene–diene monomer rubber (EPDM) was prepared with varying amounts of paraffinic process oil, such as Sunpar 2280, squalane, and dodecane. The cross-linked network composition of EPDM vulcanizates, which correlated directly with the structure and the concentration of process oils, was successfully carried out by either an exponential or a linear fitting procedure. It was found that an increase in process oil concentration caused a decrease in total cross-link density, which could be quantitatively divided into the effect of dilution and chemical effect of oil, represented as a linear and an exponential function of process oil concentration, respectively. Experimental results indicated that whether dilution effect or chemical effect, the addition of Sunpar 2280 affected the decrease of cross-link densities of EPDM vulcanizates most obviously, followed by squalane, and then dodecane, which was the weakest. On the other hand, at lower squalane loading, the chemical effect of squalane dominated the decrease of cross-link density of peroxide-cured EPDMs compared with the dilution effect of oils. Gel permeation chromatography spectra confirmed squalane could consume the free radicals derived from peroxide, forming dimers and trimers, and the variation of squalane content did not affect the activation energy of the cross-linking reaction calculated based on the Kissinger method. The cross-linking efficiency of peroxide and physical entanglement density both linearly decreased with the increase of squalane content.