If the engine keeps stalling right away then that explains why oil pressure is not building up. A failed oil pump wouldn't cause the engine to stall as you describe either.
Take your spark plugs out and inspect for any signs of trouble.
With the plugs removed, rotate the engine manually on the crank bolt and see if you can feel any obstructions as you go through a few revolutions. If it seems smooth then you might be ok.
The scenario you describe is exactly the scenario that often turns out to be the valve seat though... everything was fine one minute, then start it up and you have a hideous sound.
Anyway- if your oil pump failed, I can't imagine why you would be hearing the noise you describe. It will probably be safest to pull the head and have a look at things.