My cat used to use the litter box, but now it doesn’t, and its urine is very yellow, almost like oil.

Very yellow urine may be from low water intake or from illness. Increase the cat’s water and watch its appetite. If the urine stays very yellow and appetite or mental state is abnormal, it’s likely a disease such as liver problems or jaundice.

In that case, take the cat to a veterinary clinic promptly to confirm the cause and treat accordingly.