Half a litre of orange juice contains 270 calories. You would need to walk for an hour to burn those calories. Unless you do that, and you still eat normally, you will steadily gain weight.
Try diluting the orange juice with 50% water. That way you would consume the same amount of fluid, but only half the calories. You would burn 135 calories in half an hour of walking, and you should be doing at least that amount of exercise each day to stay healthy - more if possible.