my experience with oral cancer
I can only share my experience with you, knowing full well that each pet owner and each pet is different.
Our experience started with a loose front right fang in August, and we didn't think much of it as we've had other cats who've lost teeth around 10 years old (and we had never had an experience with cancer or any other very serious illness). But our cat Pisica was given a "fairly certain" diagnosis of oral squamous cell carcinoma in mid-November. We were told this was a very aggressive type of cancer (we never let her go back outside after this point). The vet told us the only treatment option was quite invasive facial surgery + chemo, and that this would only extend her life by a few months (2-5 months is what they told us), and that her quality of life would probably be poor because she'd be recuperating from these procedures. She weighed 10 pounds in August; she was down to 6 pounds by mid-November. We decided not to go the route of the medical treatment. The vet prepared us for the worst and within a couple weeks, by early December, she began having trouble breathing and eating; the cancer was destroying the right side of her face. By Dec 20th, the cancer had destroyed the whole right side nasal passage and she had paralysis on the right side of her face. We used small gauze pads + warm water to wipe nasal secretions and wash around her mouth. Because cats regulate their body temperature through breathing, she began spending more and more time sitting next to the baseboard heater, so I immediately purchased 2 small heating pads for her favorite sleeping spots and she loved always having a warm spot to go to. Throughout this entire time, she "acted normal"; she still expected her food on schedule, she still thought she could go outside (which she obviously couldn't), and she still tried to clean herself like she normally would (although this was difficult for her because of breathing problems), and she still wanted to cuddle with everybody and anybody she could. Around xmas time we had started giving her very mushed up poached salmon, but by new year's she was barely able to swallow any food. In the final 10 days, the vet gave us pain killer medication to be administered orally with a syringe (something akin to morphine), but our little trooper would have nothing of it. We said goodbye to her on January 9th.
Pisica was the most affectionate cat I've ever known; she wanted to be held and cuddled constantly, and we were lucky that circumstances allowed us to be with her nearly every minute in the last two months of her life. These were important considerations for us, and we don't regret the choice we made to not opt for the surgery and chemotherapy.
This is probably the most difficult part of our relationships with our pets. There isn't anyone who's in a better position or more qualified than you are to make the decision. Like another contributor told me, it's important not to second-guess yourself and simply be at peace with whatever decision you make.