The effect of the polymeric cross-link density on the thermal conductivity of an oxidized graphene (OG)-filled epoxy nanocomposite was investigated by two different fabrication methods, namely a conventional OG mixing (OG(conventional)) and a diamine-colloidized OG (OG(colloidized)) method. Epoxy composites with 3 wt% OG were prepared via the diamine-OG fabrication method to produce an epoxy nanocomposite with a higher cross-link density than that of an epoxy composite with also 3 wt% of OG prepared via the conventional fabrication method. The cross-link densities of the epoxy nanocomposites were calculated by means of a solvent swelling test. The epoxy/OG(colloidized) nanocomposite showed a higher cross-link density and higher thermal conductivity than the epoxy/OG(conventional) nanocomposite with the same filler concentration. It was observed that for the epoxy/OG(colloidized) nanocomposite, where an amide network was relatively formed between the carboxylic and amine groups, a high cross-link density enhanced the thermal conductivity by means of the transport of phonons. Furthermore, high dispersion of OG gives high dielectric constant to epoxy/OG(colloidized) even having the same amount of graphene loading.