Gray Ghost03

So I get off work walk out to my truck, start it out and my oil pressure gauge goes haywire. All the way maxed to high, I got scared turned it off and checked my oil pressure manually and it's fine. So I don't know what's wrong? My buddy said my Oil sending unit but I have no idea where that is any help please! And a explaination on how to do this please! Need my truck back up and running.