Why did my Mustang's oil pressure fail once it started being worked on?
Are you sure that you are actually losing oil pressure or is the sending unit for the gauge going bad? If the sending unit gets plugged up, or in anyway starts to fail, it can give you all kinds of crazy and inconsistent read outs when your engine is maintaining oil pressure just fine. It could also be a loose or bare wire that is causing the gauge to ground out and give you bad readings.
If you are losing pressure, its hard to say why. The oil pump being replaced may have been your problem and I would just keep an eye on that, if you no longer have an issue then problem solved. Since the oil pan should have been removed to replace the pump, I would assume it was cleaned well and any debris that may have been clogging the pump has been removed as well.
Thats the best advice I can give with the information provided. best of luck