I can't see why a scrap years ago would come back as a limp now unless there was some serious damage to ligaments, cartilage or nerves. You would have seen that slowly getting worse over time like my knees. Were any other joints x-rayed to compare them to the left knee? How old is Oreo?
What symptoms started your vet down the cancer road? I've seen too many cases where no other diagnosis fits so cancer is used as a catch-all diagnosis. Depending on the cancer Oreo needs some basic blood work, x-rays and then a biopsy. I still don't understand where the chest surgery thing comes from. And surgeons just love to operate, it's what they do! You'll be better off doing as much research as possible first; symptoms concerning eating habits, mood, mobility and what his bloodwork should be. I use PetMD and that's where I found our dogs cancer symptoms mimicked those of a gastro intestinal cyst. It turns out that's exactly what she had. Go to your vet with a well thought out list of questions, especially the "If he has 'A' then why doesn't he have 'B'? I know Oreo hates travelling but if you can get this done in one shot it will be a lot easier on both of you. With our dog we were told to just "accept it and move on" and that I was grasping at straws but you have much more time to think this through than the vet. It's not the vet's fault that in the back of their mind they know the clock is ticking. They want to care but it's a business.
What I said about BUPE. Noah is now a drug addict but so what? I have neuropathic pain and some days take enough Percocet to choke a horse. Do I want to be miserable and live to 100 or function like a human being and live to 90. I'm a special case because my liver and kidneys are 100%, I can hit baseballs opposite field, operate power tools and there's never been talk about my drivers license. Oreo isn't a textbook cat, none of them are. Maybe he doesn't need to be cut open for something that isn't there. You're his caregiver so put your foot down and expect real answers, not drama and guess work.
This is where other members with real world experience come in. Never get snooty when you get corrected and be open to other possibilities. If Oreo has some 1 in a million condition (like me) look at it this way. Whatever the odds someone always wins the lottery right?
Finally, I really do feel for you. We lost eight cats and my Mom in less than 3 years. I know it's tough but we're all here to help you get through this.