Again how big is that Jar? I have a 6.5" jar with a similar sized oscar... The O was in there first and thought he kinda owned the joint... Went around nipping at the jar. One day I woke up and the Oscar had a small hole ripped out of his side and ick all over his face... I immediately took him out and put him in a 10gallon hospital tank for 2 weeks... After he was completely healed and cured of ick I reintroduced him into the com with my Jar cats and at the time I had 2 pacu also... He didn't think he owned the tank n e more and stopped nipping at the jar and the Jar hasn't paid any more attention to him. It's been over a month and the Jar and O have been fine together though I had to get rid of the pacu for the same reason and didn't wanna go through the trouble so I just returned them to the lfs. I'd say go with an oscar as close in size to the Jar as you can... And best of luck with it all, and keep us updated with pics and all
i find that if the oscar is a little bigger than the he will be a bully. oscars are known to be bullies.
happend to me but once jar overgrew the oscar it was calmer enviroment