Vegetable matter will help if you can get him to eat it. The way we get bettas unconstipated is to feed them pieces of defrosted skinned frozen peas. If you can get your Oscar to eat some of these and do not feed him anything in the protein line plus you should check the temperature of your water. Sometimes the constipation is due to water being too cool. A fish is a cold blooded creature and when the water is too cool their systems slow down causing them to become unable to digest their foods correctly and then becoming constipated. Oscars can tolerate temperatures up to 82 degrees fahrenheit (28 degrees Celsius) and I would slowly raise the temperature to this level and allow it to stay there for a few days and see if this will help your fish to be able to digest his food better.
Do not feed him any of his regular foods for a day or so to see if he will be able to pass what he has eaten but I would try to feed him some peas if you can also.
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