Quagmire,
prad is right! You have to find a job that's related to your major. Since the lawmakers who wrote these laws have not completely defined what it means by 'related' to your major, it comes down to ultimately being able to explain it to your officer who might question you when you file for Green Card few years down the line.
Example:
You are an art major but you worked in Dunkin Donuts after graduation. The government won't care during your OPT but later on( if you apply for GC), you will have to explain some official why you worked in Dunkin Donuts after getting an art degree.
If you can't come up with a plausible reason, then you might get screwed. But say you were a computer science major and you tell them that you developed a program to manage inventory in Dunkin Donuts, that might be something related to your major. So, ultimately it boils down to can you convince someone that your job is at least related to your major without them laughing at you!