can someone please tell me who played Cousin Oscar who passed away at Archie House

Oscar was never shown in the episode.

what did Archie say about what were the words he used

Oscar wasn't shown but he was talked about and described by Archie. A big guy with red hair and warts who when they were kids would knock Archie down and sit on his face.
If he had actually appeared in a scene, I could see them casting someone like Zero Mostel, after the makeup department supplied him with a red wig and a dozen fake warts.

I wish they would have showed Oscar in episodes befor The Saga Cousin Oscar don't yall

I agree, I think showing cousin Oscar would have added to the comedy. I just loved those words Archie repeated, "what do you mean possession is nine-tenths of the law"? That was too funny!

"Oscar, you weren't happy sitting on my face, now you're gonna sit in my chair?"

for a Mintue there I did not think they were going to have a Funeral for poor Oscar