In my case the oil light and buzzer turned out to be a frayed wire to
the oil pressure switch back near the water pump. Swapped out the control
unit behind the speedo so often I could almost do it blindfolded to no
avail.

 Accidentally came across the frayed spot while cleaning the engine. It
was in a difficult spot to see just by looking casually.

 Phil Menchions
 '89 Westy