I completed “Assassinate the Assasssins” (UVHM@OP8) just prior to Sanctuary lifting off, and came back for stuff. Kinda surprised to find the gate behind Oney is locked despite the mission being done. Gonna do rest of story to see if it unlocks, but would like to know in the meantime if that’s supposed to happen. Thanks in advance for any/all help.
Oney is the second one you fight, correct? There is a known glitch where one of the enemies can be hidden from sight. Try tossing out a homing grenade.
If you had completed and turned in the mission, the whole thing resets as it’s repeatable, but it always works the same way: key gates only unlock when you’ve defeated all the enemies (including the assassin) in that area.
Never had this happen to me but I’m trying to think of what could have caused this. Did you actually turn in the mission before Sanctuary went airborne? Did any enemy respawn when you went back and did you kill them all? I just had an enemy disappear on me while doing the final round of Hyperion Slaughter Dome- couldn’t even find a red dot to indicate where an enemy might be stuck. There was just nothing- and no way to progress… 
@Carlton_Slayer @VaultHunter101 Turned it in before Sanctuary went airborne; upon returning, Assassin Wot (#1) appears with a Badass Maniac, but Assassin Oney (#2) doesn’t appear at all – the roll-up door he appears from remains closed.
Sounds like my situation in Hyperion Slaughter- it got glitched. I hope it fixes itself with a restart (and with me as well- I saved all the Slaughter Domes just to run them at OP8…).
I’ve heard of that happening before. You have to eat one or two waves before each assassin comes out of their respective spot; if any of those get glitch stuck somewhere, you either have to find them or quit/restart. I’ve seen reports of having a bandit glitched into a wall, a floor, or partway down one of the shafts. They may not always show up as red dots on the mini-map, either, but until there are out of the way, you aren’t going to be able to progress. Usually, a save/quit/restart should fix the problem, although you’ll have to start over from the first assassin if you’ve already turned the mission in.
This was for my Maya/Krieg, the only pair to finish “Assassinate the Assassins”. Both have run into the problem, so I just said “screw it” and reset the entire thing.
This is a very interesting post because a similar thing had happened to me quite a few months ago, and I remember posting it on the old forums.
It happened with my siren in UVHM lv72, I actually completed the mission and turned it in, a few days later I decided to go back and farm those assassins for the Emperor smg. Killed Wot then moved on and cleared the next area of enemies but I am still waiting for that alarm to go off with the release and emergence of Oney.
I have tried everything mentioned in this post but it just not happening. This must be some weird glitch because I have had no trouble with my other characters. A mystery for sure!! 
Must be. I tried the suggestions of homing weapons to lure/take them out – Norfleeting the gate, Homing Quasar, Tediore Deliverance – and got nothing. It’s like Oney doesn’t exist.
The gate for me is closed too, I have tried restarting the game several times but it doesn’t work. it must be something really common.
[quote=“Tomhet, post:10, topic:366027, full:true”]
The gate for me is closed too, I have tried restarting the game several times but it doesn’t work. it must be something really common.[/quote]From what I’ve gathered, doing the mission early possibly glitches Oney’s gate. I just said heck, restarted, and am gonna try that after finishing Story.