There are 2 x O.P switches on this engine, a brown 0.3bar which off memory switches open circuit at pressure over 0.3bar & is on the end of the head.
& a grey 0.9 bar switch on top of the cast oil filter head which switches to closed circuit at a maximum of 1.1 bar.
Also
Check the alternator charge rate - charging issues can set the oil warning off on these at increased revs.
Slipping alternator drive belt etc....