I just bought this van, and the oil light keeps.

I just bought this van, and the oil light keeps coming on and beeping at me! It seems to do it mostly when stopping or already stopped, however occasionally it does it while driving. I took it to the mechanic I bought it from, he told me to get the oil changed, did that. The light came back, so I took it back to him, he said he changed the oil sending unit, but the light came back on again! My question is, does it take a little while for the sending unit to "acclimate" or did we get a bad unit in there? (He has a u-pull-it salvage yard, so its very possible he got the unit used)