import annotate;
settings.outformat="pdf";

size(200);

draw(unitcircle);
dot((0,0));
annotate("O","(0,0)",(0,0));
annotate("A","(1,0)",(1,0));
annotate("B","(0,1)",(0,1));
annotate("C","(-1,0)",(-1,0));
annotate("D","(0,-1)",(0,-1));