Depending on the size of the Oscar and your tank, plecostomus are great bottom feeders but you have to get the right ones because some types are meat eaters while others are algae eaters. Some grow very large and some stay small. Considering that Oscars tend to be very dirty fish and you should have extra filtering to compensate for that, I used to keep Hypostomus Plecostomus because they grow large so an adult Oscar cannot eat it. Hypos will eat just about anything. The issue with Hypos is that they are poop machines so they will challenge Oscars for " King of the crapper" honors LOL but in a tank with good filtration, it shouldn't matter. There are other types of Plecos like the Sailfin ( P. gibbiceps) that also grow large and good feeders. If the oscar is still smaller, there are types of Bristlenose plecos that will grow to a medium size and have protection against getting eaten and are great algae eaters.
As for Bottom Feeders, there are a number different types of Catfish that would fit the bill. Raphael catfish, Pimlodellas, Acharas, wood cats, Columbian sharks, the list is long. There are catfish for every sized tank but you have get ones that are larger than your Oscar's mouth when you get it but won't grow so large that they outgrow the Oscar(s). Redtail cats are a popular fish that are sold rather small because they look so cute but they grow to over 100 lbs so they will grow large enough to eat an adult Oscar.
So you have options.