All cars burn a little oil, and all cars leak a little gasoline into the oil. If you are gaining oil, you are probably leaking more gas past the cylinder rings into the oil faster than it is burning off. If it's very thin and runny before you start the car and it's really cold out it is being thinned out by gasoline. Thin oil has less resistance pumping through the engine, so the pressure is less and the light may come on when it is not pumping hard, like at stoplights. Of course this is not really good for it.
Other guesses might be worn parts like bearings. Bigger gaps allow the oil to leak out of the pressure system more, resulting in a lower pressure.
This is all just a guess based on what you told us.
A mechanic would replace the sensor with a temporary gage so he could see exactly what was happening and give better advice