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.

For some reason, static roll nightfall loot drops at a much, much lower rate than random rolled ones. The good news is that score & difficulty don't matter. Platinum level rewards are for the shards & exotics & prisms.

I'd gauge it between 1-5%; don't know the actual results unfortunately. I'd say your best bet is to play on 750 or 1020 since Platinum ratings seem to increase your odds, and it's possible to reach 100k on the 1020 for better chances.