Sounds like its wired up correctly.
The fact that this occured right after you changed the oil could be completely unrelated.
The only way I can think of that the oil level light would turn on (when the oil level is normal while cold) is either a dodgy sensor/wiring or lots of oil is being held up in the head, or in an oil cooler. (and I mean LOTS of oil, it normally takes about 2L below normal level to get that light to turn on)
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