white spirit sometimes works, but it's better if you haven't washed it yet.
if that doesn't work try swarfega:
http://tbn0.google.com/images?q=tbn:...s/clhaswa1.jpg
you can get it in hardware stores.
rub it into the stain thoroughly and leave it for as long as you can, then scrub it out with water.
...to be honest though it might be bad news, i've ruined LOADS of clothes with oil paint, and only rescued a few of them.
you could also try painting over, but obviously you have to get exactly the same colour as your top, and depending on the material and the size of the stain, it might just look rubbish!
hope that helps, good luck!