The O2 gives the ecu the signal to run air/fuel mix for most optimum engine performance/economy based on driving conditions. Without it everything suffers. On later cars the O2 is in the cat so if you remove the cat you would need to install a older manifold for the O2. You can buy a Bosch O2 for a 86 Mustang 5.0L at any of the bb stores, Advance, Autozone, etc for around $30-35. You will need to splice the wires and seal them well. For a savings of $100 or so, not too bad. Search the forum and you'll get more info on this swap. Good luck.