1994 previa. Hard knocking when foot on brake and in drive or reverse. Replaced spark wires, rotor, and cap

If you suspect the oil pump, rent or borrow an oil pressure gauge from a parts store. remove the oil pressure sending unit (lower side of the engine by the oil filter) and install the pressure gauge. Start the engine and check the pressure reading. Should be at least 30-35PSI.

Where is the knocking coming from?

If external, I would suggest checking the engine mounts.

Oil pump would not cause knocking, especially when stopped.

Unless it caused a spun bearing.