The olive fruit fly Bactrocera oleae (Rossi) (Diptera: Tephritidae) is the major pest of olives worldwide. The figitid wasp, Aganaspis daci (Hymenoptera: Figitidae), is a larval-prepupal endoparasitoid of fruit fly species, and it was found to successfully parasitize medfly larvae in field-infested figs in Greece. To assess the potential of A. daci as a biological control agent against B. oleae, we studied the effect of olive fruit size on parasitism rates of A. daci on 2nd and 3rd instar larvae of B. oleae, by using fruit of different size (cultivar ‘Chalkidikis’) and wild olive fruit. In addition, we conducted releases of A. daci females in a pilot olive grove in Volos, Magnesia. From July to October, we released 200 A. daci females/0.1 ha/week, followed by olive fruit sampling to estimate olive fruit infestation levels and the parasitism rates of A. daci. Laboratory trials revealed that fruit size and larvae instar were predictors of parasitism success of A. daci, with parasitism rates higher for small-size fruit of the cultivar “Chalkidikis” and the 3rd instar larvae of B. oleae. In field trials, no A. daci adults emerged from the olive fly infested fruit.