Look for a round body 10-pin connector plug and socket in the wiring harness under the bonnet, above the engine. Connector may have a rubber sleeve over the plug and socket junction.
This is the connector for the engine wiring on CF2 models; the harness to the left side of the engine (right side when viewed from under the bonnet) should contain a white+brown wire for the oil pressure switch.
Remove cab engine cover for access to the engine.
If the white+brown wire isn't visible where the wires to the alternator (brown+yellow), starter solenoid (white+red) and fuel shut off solenoid (white) emerge from the end of the harness to the left side of the engine then peel back the harness tape until you find it.
Check wiring continuity by grounding the white+brown wire with the ignition switched on: oil warning lamp should light.
Switch off ignition before extending wire to the oil pressure warning switch. Disconnect battery negative terminal before working close to the starter motor and, or, alternator cables. (Same applies if you find that the dash has to come out to fix the warning lamp (e.g. blown bulb) or the 10-pin connector has to be separated.)
Connect a length of 1 mm² 14/0.03 (14 strand, 0.03mm strand diameter) cable long enough to route the cable around the back of the engine to the oil pressure switch. Ideally a length of heat resistant tubing should cover the cable where it passes round the back of the engine and close to the exhaust manifold.
Other wire in photo: no idea what this could be for apart from from a guess at converter clutch solenoid if the original automatic transmission still is fitted (which would explain the smaller diameter pipe below the coolant pipe). Try following the wire to see where it ends ...
