Oobius rudnevi (Nowicki) (Hymenoptera: Encyrtidae) is the only known egg parasitoid of the longhorn beetles Cerambyx cerdo L. and Cerambyx welensii (Küster) (Coleoptera: Cerambycidae). This micro-wasp is a gregarious and predominantly univoltine species, active during the summer when host eggs are present in the wild. Both longhorn beetles occur sympatrically in holm oak, cork oak, and Pyrenean oak forests in SW Iberia, where they can cause significant damage to trees by boring large galleries into the wood. Despite the potential of O. rudnevi as a native biocontrol agent, our understanding of its life history and parasitic capacity remains limited. This study aimed to investigate whether host egg species and oak species interact to influence O. rudnevi‘s reproductive allocation, adult size, and sex ratio. To achieve this, we compared sentinel eggs of C. cerdo and C. welensii, which were field-exposed in holm oak, cork oak, and Pyrenean oak forests over a two-year study (2017–2018). Our main findings were: 1) both host egg species and oak tree species significantly affected wasp clutch size and adult size; 2) larger host eggs were parasitised more frequently, regardless of host or oak species; and 3) host eggs with larger clutch sizes produced smaller adult parasitoids and more females than males. These results advance our knowledge of the life history of O. rudnevi as a native biocontrol agent of C. cerdo and C. welensii in oak woodlands and are discussed from an evolutionary perspective.