Styrene substitution is a major area of interest in the unsaturated polyester resins (UPR) industry and research. For most styrene-free UPRs the unsaturated polyester (UP) formulation was tailored for less toxic reactive diluent monomers (RDM). The main challenge is finding a RDM that can directly substitute styrene in industrial UPR formulations without deterioration of resin and thermoset properties. Therefore, this study concerns vinyl methyl oxazolidinone (VMOX®) as a reactive diluent monomer for industrial high-temperature UPRs. VMOX® and styrene were compared with dimethyl itaconate (DMI), methyl methacrylate, 2-hydroxyethyl methacrylate, isobornyl methacrylate, triethylene glycol dimethacrylate and 1,4-butanediol dimethacrylate. Further, the properties of produced resins and thermosets were investigated. The most striking result was comparable glass transition temperature, crosslink density, and residual monomer content of VMOX®- and styrene-based thermosets. Subsequently, DMI was applied as a reactive co-diluent for VMOX®-based resins and thermosets. Significant improvement of VMOX® conversion during resin curing was observed. In conclusion, VMOX® is an excellent candidate for direct styrene substitution in industrial high-temperature UPRs. Furthermore, the application of DMI as a reactive co-diluent improves the reactivity of VMOX®-based resins. The application of VMOX® and reactive co-diluent monomer certainly creates new possibilities for direct styrene substitution in conventional UPR formulations.