Oh Kelly, I'm so sorry to hear about Oliver, but I completely understand. You have tried everything you could with Oliver, but he just would be better as an only cat. Sometimes, no matter how hard you try, it just doesn't work out, and especially if your other cats are not acting like they used to, their quality of life isn't as good, and neither is Oliver's. You are so in tune with your animals that I know you know what's best for all of them **hugs**

This same thing happened with the older siamese cat, Sam that I had in college. She was fine in my apartment, and Aaron even brought Scooter over to see if they would get along. Sam made it clear to Scooter that she wanted nothing to do with him, but I could leave them in a room together without feeling scared for Scooter. But, as soon as we moved Sam and Scooter back to Aaron's apartment, all hell broke loose! Sam just terrorized Scooter so much that he would literally be running from her and pooping at the same time. She would just attack him for no reason at all. I think it was because my apartment was "neutral ground" and they could co-habitate ok there, but Aaron's apartment was Scooter's residence and smelled of Scooter and Sam just didn't like it. We tried everything to get it to work out, but Scooter's quality of life was just not good, and neither was Sam's. She needed to be an only pet, so we found her a good home and she's so happy now!
I know what a tough decision this is for you, so please know that I completely understand and I am here for you! I've been in the same position. It's just better for your other 5 kids as well as Oliver too.
**hugs**
Karen