Need help with serious clothing stain?

OK, I was playing soccer with my friends when the ball was kicked onto the street. I went to pick it up and put it at my side. I realized a moment too late that it was covered in fresh tar that they had put on the asphalt! Now I have a tar stain on a white and green 100% cotton shirt. We are having it soak in oxy-clean at the moment, but it isn't helping much. Can anyone please give us suggestions on this issue. If you know anything effective it would be of great help.