If I disconnect my oxygen sensor on a 1991 Isuzu Trooper (Holden Jackaroo) will my engine run rich or lean?

For safety the computer might run it rich. Leave it unplugged and see if you blow black smoke. Does it use less fuel if you leave it out? The main cause of fuel use is not running rich, but more to do with your foot. Accelerating gently, keeping the revs down, shifting up sooner and cruising in a higher gear are the best ways of saving fuel.