Vixskin is so much softer, there is really no comparison other than they both have firm inner cores! If you don't like squishy though, you will have issues with the soft, pliable outer layer of the VixSkin. It can get sticky for storage, but a light dust of cornstarch remedies this & was recommended to me by VixSkin.
Even the VIP Super Soft by Tantus which is made of only the softer, outer layer of the O2, is much firmer than I expected from reading comments on the forum! If you have ever felt cyberskin, pureskin, or any of the other "life-like" materials, that is more what the outer layer of the VixSkin feels like. The O2 just has slightly more give to it than the usual Tantus silicone.
I know people compare them, but they are so vastly different, I don't think they compare except in having a firm inner core. Feel free to PM me if you have any questions!