Well I would check your code at
Code:
if(ball._x < 0) {
oppScore++;
ball._x = opp._x - 60;
ball._y = opp_y;
ball.dy = 0;
}
because it doesn't reset the ball properly.
Then the ball doesn't really have any angle to its direction, so perhaps integrating some Math methods to give the ball a different direction based on where it hits the paddle.