Well, if you did Ostos already there's one way to clear quests to make them available again. Do a Ruins run completely and kill Falz, in any difficulty you want. That should reset the story and you would have to do the other quest to get Osto's once more.
Alright, but also when I was trying out Ostos the reason I completed it to get unsealed door was because the guy kept saying something about his stupid robot and I couldn't do anything with the enemy part(s) I had.