
The problem is Oscars so that is the dog that needs sorting.
He needs to learn that his unacceptable behaviour to Ellis will have negative consequences. If he starts when you show Ellis attention then put him out of the room. In half a minute invite him back and ask him to perform some behaviour for which you can reward him.
Next you want to be able to distract him from badgering Ellis. When he goes to have a snarl or in your face session with Ellis call him to you get a sit or what have you and give him a tasty treat.
This could then progress to an ah ah when he looks like starting on Ellis, and you can then give a command of leave when he shows you attention, and again call him to you for a treat.
The whole idea is to change his view of Ellis, that when Ellis is around something pleasant occurs for him.
Don't worry about sharing yet, and avoid petting Ellis when Oscar is in the same room.
Once Oscar's association with Ellis is more positive then you can slowly get the sharing bit in have one sit either side and treat them both for sits or downs, so that Oscar feels he is still in a win win situation even if Ellis is getting treats too.