Copoly(2-nonyl-2-oxazoline)-stat-poly(2-dec-9′enyl-2-oxazoline)s can be crosslinked by the thiol-ene reaction with glycol dimercaptoacetate. The copoly(2-oxazoline)-stat-copolyester is tested as dielectric for high-voltage applications, either as unfilled resin or as composite with nanoscaled fillers of silica, alumina, and hexagonal boron nitride. During AC voltage tests, all materials have an average breakdown strength of 45–50 kV mm⁻¹. For DC voltage tests, samples with SiO₂ (hBN) have an average breakdown strength of ≈100 (80) kV mm⁻¹, while the unfilled copoly(2-oxazoline) has an average breakdown strength of ≈60 kV mm⁻¹. Permittivity measurements at 20 °C and 50 Hz reveal that all nanocomposites are dielectrics (D = 0.06–0.08), while the unfilled copoly(2-oxazoline)s has a high loss factor of D = 8.43. This phenomenon can be retraced to the phase separation in the crosslinked copolymer, the M-OH functionality of silica and alumina particles, and models of polymer–particle interactions such as the Tanaka model, revealing that the nanofillers reduce the interfacial and dipolar polarizability.