You'll be fine with 1 pleco and 1 Oscar. I think there maybe something wrong with the water you change cause weekly 40-50% could work. What water do you use well water or municipality water?
The same water is working for my Severum tank. They're growing big and healthy. Nice colours too. I had one Oscar in the tank for last 2 years. I feel due to some reasons the cycle broke and there's no bacteria in my filters.
Termite pesticide was used in the room a month ago. Could this be a reason the cycle broke?
This has been such a healthy tank for so many years. It just breaks my heart and I feel kind of giving up on Oscars.