Hostility to new cat in home...

I took in a neutered, male from an animal rescue. He could not be rehomed as was FIV. He (Charlie) is very gregarious and when another cat (Oscar) proved FIV positive I took him and he gets on real well with Charlie.
Smokey, a female with the same medical problem moved in and everything was still harmonious.
Then came Hazel, neutered female, FIV, very shy and timid. She appears to try to mix but is very clumsy at it. Charlie and Smokey accept her but are not really interested in her.
Oscar, who is very intelligent and the boss doesn't like her. He enjoys frightening her...backing her into a corner for example and just gently batting her with his paw. She tries her low pitch growl which never puts Oscar off. Then she starts 'screaming'. This just encourages Oscar.
Oscar loves winding the others up, especially when bored. He will walk behind Smokey, as if stalking her, occasionally tapping her back legs as if trying to trip her as a predatory cat would to its prey. It never turns to anything nasty.
I have never seen him do anything but tease. However, he is frightening the hell out of Hazel.
She has nowhere else to go and is living in fear.
Hazel has been here about 4 months.
Any ideas?

Oscar continued to menace Hazel and then left. He has been gone for >3weeks. I'm sure he has not got lost or come to grief. The remaining female then started to intimidate Hazel and she has had to be returned to the rescue.
My own interpretation of what has happened is that there is something about Hazel that other cats find completely unacceptable. It is not medical. They didn't simply not like her, they wanted her driven away.
If I had respected the fact that cats know more about cats than humans do then a lot of trouble could have been avoided but I have had to witness the behaviour to learn this.

I'm sorry things didn't work out for you. I know someone who had 1 cat and brought 2 new kittens in, the original cat left home for several weeks also. She returned the kittens and original cat came home and lived happily for the rest of his life.

Originally Posted by missymotus

I'm sorry things didn't work out for you. I know someone who had 1 cat and brought 2 new kittens in, the original cat left home for several weeks also. She returned the kittens and original cat came home and lived happily for the rest of his life.

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about 6 years ago, back when I was living with my family, we had moved into a new house, I brought my old cat and we were given a new kitten from a friend who's barn cat had kittens. They didn't have much trouble, but after we brought home a new dog (who liked the cats, even though she still won't look them in the eye) he (the barn cat) stopped coming in (all my cats have always been indoor/outdoor cats). I used to worry about him alot, but found out he had gone back to the barn where his mother lived.