Oops! How did I miss this?!

I'm so sorry that things have not worked with Oliver. I know you love him very much, but its not fair to your other cats, or to Oliver, if they have to live in "threatened mode" all the time. Olver will be grateful to you for rehoming him in a place where he'll have no other kitty to worry about, and your other kitties will thank you too. You tried your best. Its not your fault. Some animals just like to be alone, and if Oliver's that type, he will be much happier in a single cat household. You are doing the right thing. Please don't be upset.