Although all evidence points to it being him here are my reasons against it being Osiris: 1) He said to us that he won't underestimate us again, to which people have said "what do you mean you know what we can do" to which I say, he's only seen us kill Panoptees, he has heard the rest but the Infinite Forrest and the actual Plain where the vex reside are very different things. So it could be a way to see if he can trust us. 2) Ikora asking Osiris "You're always plotting aren't you?" First off, what normal person asks someone that? Secondly the way he reacted could be him worried that Ikora (the person who I think is the actual spy is) had caught on to him knowing she was the spy. 3) When Lakshmi opened the portal I doubt he had the same intentions as her, but may be it was a way to secure safety to the City by having the spy (Ikora) killed in the battle which is why he left after Ikora flew in and started fighting: because he could see right then and there that no one else could see it. This is my theory and I think that it could make sense and that what bungie are doing with Osiris could be the common trope in films where it misleads you to thinking who the secret villain is.