Oil Pressure gauge

I have a 2001 Explorer Sport trac w/~80k mi. Every time I start the engine the "check gauge" light comes on because the oil pressure gauge indicates no pressure. When I first saw this I checked the oil and it's OK. After the engine has been running for about 5 minutes, and the temp. gauge gets to about half the normal point, the oil pressure gauge springs up to normal. I wondered if anyone had had this problem and/or if they had an idea on what the problem was??