AbstractOobius rudnevi (Nowicki) (Hymenoptera: Encyrtidae) is a gregarious egg parasitoid of Cerambyx cerdo L. and Cerambyx welensii (Küster) (Coleoptera: Cerambycidae), two well‐known large oak‐living saproxylic beetles with different pest/legal status: whereas C. cerdo is a protected species, C. welensii is considered an emerging pest. Despite the potential interest of O. rudnevi as a native parasitoid, the knowledge of its biology remains very limited. We used sentinel eggs to assess the distribution of O. rudnevi in Extremadura (southwest Spain) and to explore its ecology, phenology, behavior, parasitism rates, and host preference. More than 13 500 sentinel eggs, housed in 1 030 wood disks, were field‐exposed during 2016–2018 in 147 oak stands over an array of oak forests. Oobius rudnevi was the only egg parasitoid detected, being widespread and relatively abundant over the studied area (90/147 stands, 61.2%) with adults flying in May–July. Parasitoids were detected in both host species, but net parasitism (PN) was significantly higher in C. cerdo (22.4%) than in C. welensii (9.5%). Net parasitism was affected by altitude (it was higher at 600–800 m) and tended to increase in colder aspects; PN was poorly related to host egg density (no. eggs per disk) and unrelated to host population size (no. adults per trap). Most parasitoids overwintered and emerged in the following spring (synchronously with their hosts) but some phenotypes/lineages did not undergo obligate diapause and were bivoltine. Bivoltine wasps were spatially widespread (27/90 stands, 30%), even if their actual prevalence was rather low (60/1641 parasitized eggs, 3.7%). We also found that O. rudnevi was slightly protogynous, the sex ratio was female‐biased, and the mean (± SEM) number of wasps emerging per parasitized egg was 9.3 ± 0.1 (range 1–29). We discuss the potential role of O. rudnevi as a biological control agent from an evolutionary perspective.