I don't think you need to worry about gaining weight from the olive oil. I'm pretty heavy myself and got into this because it didn't make sense to me that I'd lose no weight at all on the same diet my boyfriend dropped 30 pounds on (I was the cook, after all, so I know it was exactly the same, plus I was walking and he wasn't). So I did some serious research and wound up here. Now I don't worry about the weight, I just eat right and flush and figure it will inevitably come off when my body is working better. Already my skin is better and energy level is better, I sleep better and feel stronger and haven't had the usual pollen count problems this summer (6 flushes so far, mostly chaff and small stones, a few larger ones).
Getting back to the olive oil, I look at it this way. You eat a light breakfast and lunch, then you fast, the next day you probably don't eat until nearly noon and that's a light meal too. 8 ounces of olive oil is 1600 calories. You make up for most of that by skipping dinner the night of the flush and eating light before and after. So there really aren't many extra calories. :)