Well thats a tough one.... To find cichlids that will do well with the oscars is very tough. Especially if the oscars are already in an established tank by them selves. They are very highly territorial and once they have claimed a tank there is no humane way to add additional fish. In addition, they are very dirty fish, so they will put a big bioload on your filter, so adding fish will also increase that; make sure you have a big enough filter, or add an additional one. Also they are littlle pigs, as you probably know!!! They are so eager to eat food that often tank mates don't get any, and so they do not do as well. The larger the oscars get the more pushy they are going to get.
You may have better luck adding some Non-cichlids that are as large as the oscars. Something that can take a beating and keep on ticking!!! Tin Foil Barbs are ones that come to mind! Also silver dollars might be an option.