my serpentine is slipping and I assume at this point, I need to replace my tensioner pulley and belt. but I also lost engine oil pressure.

I killed the engine, tried again and had low pressure, about 40 indicated and so long as I kept the RPM low and a low load on, I maintained pressure. if I revved it as if driving normally, it would drop to 0. I limped the truck home on lower pressure. I don't remember at what RPM I lost the pressure.

anything I should look for with the oil pump before just swapping in a new one?

Maybe it's just the sending unit. Check your idler pulley, if it doesn't roll smooth, replace it also.

Put a mechanical gauge on there, and see what oil pressure is REALLY doing.... The sending units are notorious for failure though....