If something was offered by the factory as an option, that something can be added to the airplane, using factory parts, with no STC required.
If your Skymaster had O2 as a factory option, which I believe all did, you can find the factory plumbing, new or used, and install it with nothing more than an A&P sign off.
Make sure your O2 bottle meets aviation standards, and has been hydro tested recently. There may be some lightweight carbon fiber bottle options out there, I'm not sure what paperwork they require.
Your best option is to sell it and buy a "P" model.
