With 7 Oscars in a 240, you're looking at a minimum of an 85% water change every 3 days. That's to keep nitrates in the 15-25 range which isn't great.
If you're planning on breeding, I can't tell you what their eggs would do, but in studies the eggs of other species have a very high mortality rate at anything over 3-5 ppm.
Also note that Oscars are notoriously dirty eaters and water changes might be preferable more frequently simple because of the excess debris that might be in the tank. Again, I haven't kept 7 at one time, but I would suspect the tank will need a lot of cleaning.