Oka is definitely dead. There is no doubt about it. Zoom in on the picture (http://www.onemanga.com/Gantz/276/14/) and you can clearly see that the two outstretched fingers are ATTACHED TO HIS LEFT HAND along with the bent pinky and the thumb.
In Nurari's right hand is Oka's lifeless torso and a half-broken spine. All the more proof that the "strongest man on earth" is in fact, dead.
For all those who stubbornly argue that for some blue-hell bizarre reason Nurari is trying to *FAKE* out Katou into thinking Oka is dead, just re-read the chapter again; or save the picture and zoom in enough to see it. I mean the whole theory of "faking" out Katou is already dumb enough as it is, so please try not to misguide anyone else who's having doubts.
If you are such a fan of Okas, pray that the mangaka will bring him back via the 100-point resurrection, because there ain't no damn way Oka's coming back home like Jesus rising from the graves.
Lastly, for those people who says that the body would be seen: notice how nurari lands with an angle, or he is being propelled forward. Basic physics would prove that the body that is held in the right hand would swing back from the force of the propulsion, aptly being covered enTIRELY by the 2 arms, which completely covers the background (in the sense that you can't see the background in between the forearms). For those who thinks that the body is too thick to be covered, notice the way he lands: SIDEWAYS. Which would imply that Oka's torso would also be swung back SIDEWAYS, which is indubitably hecka lot thinner than the frontside view.
POINT AND CASE PEOPLE. Nothing to see here. Story's over. Go home.