Oil gage

Check your oil level and make certain you are full. Then it gets harder to trouble shoot. I would start at the oil sending unit that provides a signal to the gauge. The gauge measures oil pressure, not whether you have a full system, so if you are not low on oil, something is causing the oil pressure sender to measure erratically or you have a loose connection, After that and the problem is still evident, I would let a shop look into it. They have the tools to evaluate the sending unit and the gauge efficiently.

You can also purchase a gauge to check the pressure for twenty to thirty bucks at your local parts store. It will be a direct pressure check off the engine. You'll just need to look up the correct pressure reading for your vehicle on the net.