OH NO! I'm so sorry. I've never heard of a dog so young having osteo.
I lost my last senior dog to it, in 2013. He had it in his elbow, and amputation wasn't possible because he already had pretty bad HD in the rear and couldn't support the extra weight.
You need to get an appointment ASAP with the oncologist. Just make the appointment now and wait for the referral paperwork to come through as they tend to be pretty solidly booked. That way, by the time you get whatever paperwork you're waiting on, it will be time for your appointment. Here's a directory of them:
Back when my dog had it, the best treatment option was palliative radiation, which takes the pain away and slows tumor growth, but doesn't cure it. It gives them more quality time without pain. There was no curative option for us. I think the radiation treatments are around $3,000 -- plus more if you add acupuncture on treatment days.
It's a VERY PAINFUL cancer. The most important part of the process is giving your regular vet the job of monitoring your dog's quality of life and pain management, and telling you when it's time to stop fighting and let the dog go -- the oncologists tend to drag out the fight with everything available to them, as that's their training. Fight the cancer for as much good, quality time as you can get the dog, but once you get into the fight, it's very important to have your regular vet in your corner to protect your dog from suffering and tell you when to let go. Making that decision is the hardest thing in the world, and we're often not in a good frame of mind to make it with all that going on -- so have the conversation with your regular vet as soon as you engage the vet oncologist. I actually created a group email and put the regular vet, oncologist, and holistic vet doing acupuncture all together and let them know my regular vet was "quarterbacking" the case, so they needed to keep him in the loop so that he could oversee everything.
Please order yourself a copy of the 2nd Edition of the Dog Cancer Survival Guide
by Dr. Dressler & Ettinger (a holistic vet and vet oncologist in dialogue). It was invaluable to me to read before our appointments, so I could go in with questions and better understand what the oncologist was offering as options.