Get a voltmeter on the indicator light circuit or read the voltage off the light base itself (if it's incandescent as I suspect a 23 year old lamp would be).
Find out the output of the low oil pressure sensor, or (my choice) replace the sensor with a 12 VDC one, and then you can run the audible alarm in series with the lamp, or just get rid of the lamp, assuming you don't have deaf helmspeople.
Another possibility is that you've either wired the buzzer incorrectly or that the amperage is insufficient to activate the buzzer. Try it with the lamp OUT of the circuit.
Which makes me think...how do you create the conditions for oil pressure low enough to light the lamp in the first place?