Jim,
undergravel filters and adult oscars don't go well together. Actually your idea of plumbing it all to the sump would be ideal. Oscars start to dig in the substrate when they think they are grown up and want to start housekeeping. all that digging and rearranging of the tank decor and life support system will completely undermine the efficiency of the filters and could lead to fried (or boiled) fish, broken heaters, broken filter tubes....you get the picture. The ideal is to get all the junk out and into the sump where it is safe. As to filtration, you'll need excellant bio filtration since oscars are not only big (up to 15") but they are also gluttons with sloppy table manners. Run the water from the tank through pans of sand, over bioball, through sponges, over biowheels, or, all of the above. For ease of cleaning, I would get rid of gravel in the main tank and replace with slate on the bottom and large rocks and bogwood for decor. It can look really sharp and be easy to keep clean. For the ultimate bottom, select some really nice slate, or even marble floor tile and glue (with aquarium sealant) to the bottom for a permanent floor that detritus will not get lost under. If you want a great read on keeping big fish, look for "The Manual of Tank Busters" by Sandford and Crow by Tetra Press.
