Onlay bone grafts have a bad reputation of resorption with loss of contour and volume. Rigid fixation reduces the incidence of resorption but does not prevent it. Literature shows reduction of resorption by applying guided bone regeneration (GBR) barriers and platelet-rich plasma (PRP). Investigating the effect of combining them together to reduce resorption was the aim of this study. This study included 4 groups: control group, GBR group, PRP group, and GBR + PRP group. Twenty rabbits were used (40 mandibular halves). Onlay bone grafts were fixed by titanium miniscrews in all groups. Computed tomography scans of harvested mandibles after euthanasia allowed calculations of bone graft volume and density. Onlay bone graft volumes in all experimental groups were significantly higher than in the control group. Volume maintenance in the GBR group was significantly higher than in the PRP group. There was no significant difference in the volume of onlay bone grafts between the group of combined GBR + PRP and GBR alone. It was concluded that, to maintain the volume of onlay bone grafts, either GBR or PRP can be added. Combining them did not add any advantage over the GBR alone.