Have you talked to your vet about ORS and the quantity? What about sub-Q fluids, administered under the skin vs. an IV drip? If shown how to do it, you could give sub-Q fluids at home at the vet recommended quantity and timing.
I have had a cat with kidney disease get subq fluids at home 3x a week for 2 years. I would ask to learn how to do this. It is a lifesaver. If dehydration is the issue that will fix it.
I don't know what ORS is. But subq will work great for dehydration.
As cats age, they can develop chronic illnesses like kidney disease. Dr. Lee discusses the benefits of administering fluid therapy to a cat living with chronic kidney disease.
We can't diagnose for you; if you are going to proceed with oral rehydration solutions administered intravenously, a vet has to give you instructions. I agree with others that you should ask about subQ fluids for your cat....also verify amount and procedure with the vet.