Most commercially available olive oils are virgin or extra virgin oils, rather than refined (chemically treated) oils. Virgin oils are comparatively higher in antioxidants than refined oils. By contrast, most commercially available grape seed oils are highly refined and, as such, contain smaller amounts of antioxidants. It is important to note, however, that an unrefined grape seed oil, one that has been cold pressed and not chemically treated, is very rich in antioxidants.
Also, some people confuse grape seed oil with Canola oil. They are very different oils and come from completely different sources.