While there are different body compartments of water to deplete in order to cut weight, the way that I have found the most success with athletes is to deplete their glycogen stores specifically.
To do this you simply need 3-4 days before the competition (and hopefully you have tried this before the competition to figure out how well your body responds to it as some people don’t do very well on low carbohydrate/glycogen stores). And, because O-lifting events make you weigh in only hours before your weight class competes you have very little time to replenish (luckily for the sport high glycogen levels are not necessary).
It takes a little bit of calculating and you need your body composition numbers too.
Your muscle tissue is about 2% glycogen by weight, so if you are a 90 kg lifter at 10% body fat (estimated by shoe of course), you have 81 kg of lean tissue weight (estimated). This means that you have 81kg x 2% = 1.62 kg (roughly) of glycogen on board in the muscles. Since for every gram of glycogen you have 3-4 grams of water associated with it meaning 1.62 x 3 = 4.86kg of water that can be lost from depleting those stores completely. How much you actually lose will depend on a couple of things including how well those stores are depleted.
Now some people (women especially, seem to be able to deplete quite a bit and feel fine, but others can’t as they simply feel crappy.
The best and really only way to deplete them well is to as Koing mentioned eat a low carbohydrate diet (I usually start this 4 days out from the event by cutting carbs to 50 or so grams per day, only eaten around training time).
Then I have the athlete watch their weight from 4 days out to 3 days out to 2 days out. At this point the athlete usually is losing weight at a reasonable weight, but if it is a big competition and they are not training very much (bc they are tapering), they won’t be activating their muscles very much and hence won’t be using the glycogen very much. So I have them do some brisk walking for about 1 hour 2 days out and the day before. This should roughly be around 55% of heart rate max (220-age * 55%) so around 110 to 120 bpm.
The exercise is key as you only really use glycogen in the muscle to a large extent when contracting the muscle.
The method outlined above is pretty good overall, there are more accurate ways to estimate total muscle mass, and even dietary ways to dial in the glycogen content, but the above works pretty well.
That usually does it for most.
Hope that helps