We haven't seen Oryx's powers yet, but we know two crucial things about Atheon. 1. His main ability is the control over time itself, and he has the power to send things into the past or future where they can't inhibit the goals of the Vex. 2. His power is drawn from the Vault itself. If it weren't for that fact, the Vex could have simply unleashed him and the Gorgons on Earth and wiped everything out. Theoretically Atheon could send Oryx into the future, where he would presumably be helpless against the might of the advancements the Vex have made since the present day. However since Atheon can't leave the Vault, he is limited in ways that Oryx isn't. We don't know if the Vault can be destroyed but we saw that the Wolves managed to bust into the Vault on their own without much effort. So if Oryx decided that Atheon was a threat he could simply crush the Vault with firepower and never have to face him at all. So without knowing Oryx's true strength but seeing Atheon's limitations, I'm gonna go with the Taken King as the winner.