Oranges are low in fat and are healthy for you to eat and are also good sources of fiber.
That said oranges do have calories and all calories can contribute to weight gain if you eat too many. Orange juice on the other hand tends to have a lot of empty calories.
Oranges are very high in sugar just like bananas are. They go to your pancreas much faster than other fruits. When they get there they turn into insulin which then turns into fat. If you must drink OJ and your heavy add some protein with i.e. cheese,cottage cheese and some monosaturates i.e. 2 nuts or olive oil or avacado. This slows the OJ down and when it arrives to the pancreas it is "used up" and does not turn to fat. For more info on Carbs vs Proteins and Monosaturates Look up the Zone way of eating and eat more food and lose weight and have more energy than you ever had. Also only eat a half banana at a time. "The Zone Diet" by Barry Sears. You'll lose weight while you read the book. Seriously