Now before I start this I have no in-depth or leak information leading to my assumption of Osiris in any way, I am just going off of what everyone can access. It may seem extremely obvious, but I believe that Osiris is an Awoken. Since his Trials are beginning once the House of Wolves DLC, a raid centered around Fallen and AWOKEN, it makes sense. But, I also looked at some of the other activities in the game and this just seems possible. First with the Vault of Glass, we are told of a brave Titan, named Kabr, which was involved with a Raid surrounding Vex. It seems fair to say that Kabr could possibly be Exo since both are related by the idea of robotics. For The Dark Below, Eris Morn may be Human. The only two places the Hive inbabitat are mainly the Moon and a little bit on Earth. The only other group that inhabitats those planets are the Fallen, and the Guardians of course. Say we've already crossed off Exo from the list, that leaves Awoken and Human. The only race, in the real world outside of destiny, thats visited Earth and the Moon are Humans. Finally with the House of Wolves. We are centered around the Fallen, who have beef with the final race, the Awoken. It makes sense looking at everything else this game has given us.