This sounds like you have a bearing clearance that is excessive enough to cause the oil pressure at low engine rpms (when the oil pump's output is lowest) to fall below the oil sending unit pressure threshold for when the light turns on. That threshold is often in the 7 psi range.
I would try missleman's suggestion for a new sending unit first.
If it were my car and a new sending unit didn't solve it, I'd put an oil pressure gauge on it to get an accurate reading for what the pressure really is. If it is only low at idle but rises to "safe" ranges at higher rpms, then I'd live with it, perhaps with a slightly thicker oil. Many engines have had long lives like this.