Why is my older cat straining and using the litter box so much?

Poor Oreo! Straining in the litter box can be caused by constipation, diarrhea, or urinary issues - it's often quite difficult to tell if a cat is straining to pee or poop, so it can be hard to differentiate these things if you aren't used to looking at them.

The most concerning possibility is that Oreo may have a urinary obstruction, especially if you have not seen a significant amount of urine in either box since yesterday. This is unfortunately a very common problem in male cats, because their urethra is extremely narrow where it exits the penis - this area can often become "plugged" with crystals or mucus from the bladder. Signs of this problem include repeatedly going to the litter box without producing much urine, crying when trying to urinate, and often lethargy, decreased appetite, and vomiting. If he does have a urinary obstruction, this is a life-threatening emergency that needs immediate treatment, as the bladder can rupture and cause death within 24 hours in some cases.

It's also possible that Oreo is using the litter box so much because he has diarrhea, or is having trouble passing stool because he is constipated. These problems are less common than urinary issues in my experience, but would still need to be treated by a vet - especially since he's been acting this way for two days now, and seems to be getting worse.

I would recommend taking Oreo to the vet today for an exam. He/she can palpate his abdomen to check his bladder and colon, and may also recommend some diagnostic testing if needed to help diagnose the problem.