After the oil change, the light did not come on for a couple of days and now it comes on about half the time. Does the light primarily come on when the engine is idling after warming up? If that is so, it does not mean that no oil is flowing or there is no oil pressure, it means that the oil pressure is below 7 psi.
This is typical for an engine that is pretty much worn out. I would have your mechanic check it out because it could be something as simple as a leaking oil pressure sending unit. Have you been losing oil between oil changes?
If you have an oil leak that is causing this, you should get it fixed. If not, then you may have to make some decisions. Is the vehicle worth putting a remanufactured engine in? If not, and there is no simple repair, you might try using a heavier weight oil, like 15-40 or 20-50. A 15-40 Diesel grade oil such as Delo or Rotell would be my choice as those oils still have the recently banned anti-wear additives that would provide some protection during the low oil pressure times.
A good check by a mechanic will take some of his time, be prepared to compensate him for this, but it would be money well spent. Go to a independent mechanic with a good reputation, you will get a little more personal attention on this.