If anyone comes across this still needing the Osprey, it is very possible, if totally up to random chance: I played through the 750 light version of the Nightfall Ordeal 37 times over the past few days before I got it to finally drop. If you have bad luck trying to get this weapon just keep playing the easiest version, and don't lose hope. It's literally a war of attrition against the horribly low chance that it can spawn.