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My name is***** and I can try and help with Oliver! Are they both spayed/neutered?
At this age, they are reaching social maturity, and they can start to display some different behavioral issues. It is not uncommon for this to happen after one of them startles the other one.
You can try to correct his aggressive behavior by feeding them treats or petting them while they are near each other, to reinforce that good things happen when they are close to each other.
Something else that may help is to get some Feliway, which you can find at the pet store. It is a phermone that helps to relax them, and it comes as a plug-in and a spray.
Sometimes cats act more aggressive when they are not feeling well, so it is important to make sure Oliver is eating and urinating normally.
Do you still need assistance?