while driving my dads 2000 SE the oil pressure skyrocketed to 120 PSI for no apparent reason, what could be causing this to occur?

Oil pressure sensor needs replacing I'm assuming. If you were actually at 120psi, you'd be blowing oil out passed the weakest oil seal.

the sender went bad... $30~40 part that you should replace next time you change the oil

^ Ok you can call it a "sender" if makes you feel that much better.

would it cause harm to ignore it?

probably not.. but id suggest getting it taken care of asap.. cuz if your pressure does drop or rocket, you wouldnt know

My oil pressure gauge goes to 120 everytime I start the car and remains there until I turn it off. Is that a problem?

Apparently you have the same problem as the OP. It'* supposed to be right around 45 psi - 55 psi with the oil hot and around 65 psi - 70 psi with the oil cold at startup. :o

Ok I checked the ALLDATA freebie and apparently the sending unit is just around the corner from the crank pulley facing aft towards the firewall. :o

Yeah, it'* not necessary for the sending unit to be replaced, but it'* like having no fire alarms in your house. Sure, as long as there is no fire, you don't need the alarm. But what about when the christmas tree lights up at 3 AM?

I've ran my truck with a bad sender befroe, and nothing bad happened. The only ill effects are the lack of knowing where your oil pressure is at, and that can be bad. Get it changed sometime soon.

There is a potential problem that could develop if it is not replaced. If it fails all together, it will cut power to the fuel pump leaving you stranded. It'* an engine safety feature so it doesn't get destroyed by running without oil. Murphy'* Law would likely come into play.