To answer how Oryx’s remains ended up on Titan there's a few theories. Lore-wise there was two years between everything that happened in D1 and everything thats happened in D2. When we killed Oryx in Kingsfall, his body went floating off into space. It's entirely possible between gravitational pull (Titan is Saturn's largest moon after all) his remains orbited around Saturn and eventually pulled down into Titan's depths. The Last Days on Kraken Mare lorebook, gives us an idea on what happened to Titan during the collapse. The result of which has altered Titan’s gravity as well as underwater landscape and well the Vanguard has had more pressing concerns than determining what became of Oryx’s remains. Or the Vanguard deemed the retrieval of the remains was unfeasible or simply too dangerous to attempt. We also know the Hive have been on Titan for awhile. Titan has kinda become akin to a war-moon (not unlike what the Hive done on Luna) which the Hive use as bases of operations and places prepare troops for their invasions. So it's possible Oryx used Titan as a rallying point from which he could stage invasions further into Sol prior to the Battle of Saturn. Furthermore it's possible there were Hive on Titan who were able to intercept Oryx’s remains and bring them to Titan. Or perhaps Xivu and her forces where responsible for their retrieval and bringing to Titan.