1979 Oldsmobile Delta 88 wont start

"Doesn't start" can mean a lot of different things. Thank you for adding "nothing happens". So the starter doesn't crank the engine. The next thing is to listen for any clicks or relatively loud single clunks each time you turn the ignition switch to "crank". If you hear absolutely nothing, suspect the neutral safety switch first. Trying starting it in neutral instead of park. If that works that switch is either defective or the shift linkage is slightly misadjusted.

If you can hear a light single click from the starter relay, the contacts in that relay could be arced or corroded, or there's a problem with the solenoid on top of the starter. The fastest test is to use a test light down at the starter. You should find 12 volts, (the test light will light up), on the fat wire all the time. There will be two smaller terminals to either side and right below that fat wire. One of them must get 12 volts when a helper turns the ignition switch to "crank". At the same time that 12 volts on the fat cable must not disappear.

Holler back with what you find and we'll figure out where to go next.