Oil light is on and engine knocking

Just purchased a 2000 Leganza with 38,000 miles, the first prob was leaking oil from the valve cover which was an easy fix but then my oil light came on and i noticed a knocking noise coming from my engine and then i loose power when ever this happens. I would say the light usually comes on after the car warms up. What could the problem be? oil preassure switch? oil pump? piston rods?

oil pump,dont drive it till it is changed..