Melted chocolate + olive oil = lumpy mess
That happened to me with margarine instead of olive oil. My question is here, but basically Aaronut told me 3 reasons why chocolate seizes.


  
  Using too high a heat. Double-boiler is the safest, but you can use a saucepan on very low heat.
  Sugar bloom and other impurities. You shouldn't get this with baker's chocolate, but if you use any lower-quality chocolate, this can seep into the melting chocolate and cause it to seize.
  Contact with moisture! Even a tiny amount will cause it to immediately seize, and it's difficult to recover at that point.