Oryx cannot come back, he was defeated in his own throne world. No ascendant Hive can survive the death of their true selves in their own throne world. Both Oryx and Crota were killed in their own respective throne worlds. Their true selves, will, and power were defeated and destroyed.

Defeated isn't the same as being killed. He just loses his grip on the Dreadnought and floats away towards Saturn. It doesn't seem like he needs to breath, so I don't see why he wouldn't survive falling to Saturn or one of its moons.

He was killed within his throne world. The Dreadnaught is his throne. As he says himself. [quote]If my echoes are killed, and I am killed in the material world, then I will be driven back to my throne the Dreadnaught. If my Court and my throne can be beaten, if I am confronted in my throne, if I am defeated there, then I will die. My work will end. [/quote] His mortal form was killed and he was driven deeper into his throne world. Both Oryx and his Court were killed in the raid. The worm gods tell him [quote]From this day forward, Auryx, you and your sisters will each survive death — so long as you aren’t killed in your own throne.[/quote] Oryx and Crota both died the final death.

He looked still alive to me when he was floating away. If not alive, his body was at least intact. Maybe another worm will posses and animated his body. Not that I would want to fight Oryx again, but they did seem to make it a point to show him floating off instead of disintegrating like most bosses.

If he was alive, then his worm would devour him. [quote]This is the pact to which I am bound, in particular by my study of the Tablets of Ruin, and by my use of the power of the Deep. When I call upon that power, I put myself up as the stakes in a wager, I gamble with my soul. For I am saying, listen, my gods, I am the mightiest thing there is, and I prove it thus.[/quote] We proved that he was not the mightiest, he lost the gamble. I would imagine that someone checked the status of Oryx. Eris certainly would have verified that he was truly dead. We also used his heart to make the touch of malice (not his worm as some claim).