I think the ornate would be fine with the senegals. I would worry more about lower jawed polypterus than upper jaw polypterus. Personally, I've never had a canniblism incident, and I currently have a 16" ornate with a 6" Del (and lots of other species in sizes in-between) in one of my tanks. Although, I do keep a 3" or so albino semi-SB in a grow-out tank, and I wonder if he'll catch up.
I think it'll be harder for the ornate to keep up growth along with the oscars. Oscars tend to eat larger quantities much faster than polypterus, they process the food faster and experience rapid growth.