amber oil warning light

The oil light should come on with ignition, then go off only when the engine starts. The last time I saw an oil light with an apparently ok oil level was on my old Vauxhall Cavalier, whose engine seized approximately 100 miles later. One of the oil channels was blocked so the top end wasn't getting the oil, but the sump stayed full and gave the impression of 'enough oil'. I needed a complete replacement engine for that one...

I would get it checked out ASAP, even to the point of not driving the car.

Can you confirm which light - the oil light is fourth along (out of five) on the top row of the fifteen annunciators in the square panel in the middle of the dash panel. Has an old-fashioned oilcan with a drip coming out of the spout.

RTFM
From my copy:

11 - Engine oil pressure/level
The warning lamp lights up for a few seconds as a functional check when ignition is switched on.

If the warning lamp is red or yellow or flashes it indicates that the engine oil pressure or level is too low. This warning lamp will also light up if there is a defect in the oil level indicator.

If lights up or flashes red (insufficient oil pressure)
If the warning lamp lights up or flashes red while driving an acoustic signal will sound three times at engine speeds above 1500 rpm. Stop and switch engine off: check oil level and add oil, if necessary. See page 3.41.
If the warning lamp flashes even though the oil level is in order, do not continue driving. Do not even run the engine at idling speed. Call in expert assistance.

If while driving the engine speed decreases below idling speed the oil pressure warning lamp may light up. Increase engine speed by accelerating or changing down gear.

Lights up yellow (oil level too low)
The yellow warning lamp indicates that the oil level is too low. Stop engine and add oil- see page 3.41.
When opening the bonnet the oil level warning is reset. However, if you do not refill oil the warning will reappear after about 100 km.

Flashes yellow (defective oil level indicator)
If the oil level indicator is defective an acoustic signal will sound and the warning lamp will flash several times.
From the moment of the defect to the check-up of the engine the oil level needs to be checked regularly, preferably when filling the tank.

Your symptoms match the low oil level diagnosis. If the oil level is not low, then there is a fault with the sensor which is in the bottom of the sump. Check the wiring and the sensor.

You need to get this fixed, even if it's a faulty sensor or wiring, it could mask a more serious fault indication leading to the destruction of your engine. I'd never be happy driving a car with an oil light on.