What's the difference between: oil pump failure & the oil warning light error?

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I just had the oil changed on my 1997 Plymouth Grand Voyager 4 days ago. It ran like it usually does with no problems. Then last night when I went to use the van the Oil Warning light came on intermittently. I checked the oil level in the engine and it is full. On another site there was a warning that the oil pump can fail so I don't know if the problem is an oil pump failure or the "idiot" oil warning light sensor is failing. There are no unusual leaks or noises from the engine. There has always been a light tapping when I first start the vehicle. Sometimes when I press the accelerator, the oil warning light goes out, but when I come to a stop it pops on again. The Van has 103,000 miles on it. I drive less than 2,000 miles a year because I am disabled. Any help you can give me is greatly appreciated! Thanks so much for your help! Sincerely, Bob