I'd try a large amazon sword . . . if possible, find one that is already potted, and then put the whole thing -- pot and all -- into the tank and bury it well. then place some rock around the sword, preferably some flat rocks with large (3"+) diameter . . . no guarantees the Oscars still won't rip up the leaves, but it will make it much harder for them to uproot the plant
you could also try some onion plants (crinums) . . . they are tough as nails, easy to plant (and replant), and since they taste awful, fish tend not to eat them
lastly, you could try anubias; tie them down to heavy rocks and/or driftwood; you can use fishing line. they are very tough, Oscars won't eat them, and if you tie them down properly, they won't be able to pull them off the rocks either