Who, or what is a Keith Williams?
Oil pressure is sampled at some point in the pressure line downstream from the oil pump. Generally it's not measured at the oil pump outlet or inlet, and generally the oil pump is internal to the engine, or mounted as an accessory.
The location where oil pressure is sampled depends on the engine in question. Oil pressure may be sensed in the flow line from the oil pump, or at some other point downrange. In piston engines, oil pressure is generally sensed from a passage point in the block, while in turboprops it may be sensed along with torque at a point in the propeller gearbox, or elsewhere in the pressurized supply. Turbojet sense works the same way, and may involve more than one pressure sense point. In fact, often a low pressure light is triggered by a different sensor or switch than the oil pressure indication, and a loss of one or the other doesn't necessarily mean one has lost oil pressure, or has an emergency.
Temperature sensing, conversely, is often placed at the oil cooler outlet, or near the pump inlet, though with some systems it's also placed in the oil cooler inlet, too. This can be significant because if temperature is sensed at the oil cooler outlet, one can only see the temperature of oil leaving the cooler. Oil bypassing the cooler with a stuck vernitherm or thermostat won't be sampled, and one may have a false indication of temperature that's artificially low. Sensing in more than one location is also sometimes used.
Oil pressure is generally not measured right at the pump outlet because it doesn't give a true measure of the pressure that the engine is receiving. To protect from excessive oil pressure and to regulate oil pressure, a means must be provided to increase or decrease oil pressure; this is generally done using a pressure relief valve or bypass valve. The way this works is a valve opens at or just after the outlet side of the oil pump to allow excess oil to be sent either back to the oil tank or sump, or commonly back to the inlet side of the oil pump. Measuring oil pressure downstream of this bypass point, then, gives an indication of the oil pressure that the engine is actually receiving or using. That's the pressure that interests you as the operator of the engine.