Male cat being ostracized

I have 4 cats; about 6 yrs ago we took in a feral cat from a friends yard. His name is Ollie, after he acclimatized he became a loving cat but still a little skitish of people and will hide under my bed. I worked hard with Ollie and he has been a wonderful cat, he is considered 'My Cat'I went away for 4 mths and upon returning the 'baby' cat Guinness who is 2yrs old has completely pushed Ollie away, so now he stays under the bed, Fortunately I am at home all day, how do I stop Guiness being mean and hissing, and reintroducing Ollie back to the group?

Hi @Antiquejules, welcome to TCS and sorry for the late reply!

Poor old Ollie.. I feel for him.  Others may have a better idea, but I think a reintroduction may be the way to go.  Even though it's a long, involved process it could well get things back on an equal footing.

If you want to try a quick fix,  how about a Feliway diffuser? It may just settle Guinness, but there's no guarantee.