Welcome to PT. It sure does sound like Oscar is very bored. Some cats need a lot more attention than others do. I currently have 6 cats in a fairly small space too but only my kittens Alani & Blaze are the ones that are constantly getting into trouble. How old is Oscar? If he's still young, hopefully he'll mellow out as he gets older.

I've had to put child safety locks on all of my lower kitchen cabinets and bathroom cabinets. I have to keep all paper and plastic products locked up. I always keep my shoes in my closet. Both of my bedrooms are off limits for Alani & Blaze because I have two computers and many other things that they can get into.

Most cats hate the smell of citrus or even minty things like toothpaste or mouthwash. You could try spraying a citrus spray or even putting some orange peel around an area that you don't want Oscar to be in. Sometimes double sided tape will work too because cats don't like sticky things.

Maybe you could also try getting Oscar a toy that you can put treats in and then he has to figure out how to get them out. I think you can also make something like this. I hope that you'll be able to find a good solution soon. Good luck.