Drinking a lot of water, salads, lemonade and proper sleep would help induce digestion during the OMAD diet. Eating fruits help add fibre to the human body and adding lemon juice to the one time meal would help in preventing constipation.
Water melons are good source of natural water and fibre, eating tender coconut jelly and drinking tender coconut water also helps prevent constipation.
I usually take vco when I feel I'm constipated. I take two teaspoonfulls with a glass of water and the effect is almost instant. I believe that there are really some foods that can cause constipation as there are foods that make our poop soft.. It would be best to probably avoid foods that constipate. Another thing I take when constipated is prunes. Just be careful not to take in too many of these black goodies because you'll end up having diarrhea instead.