Each Medicare Part D / Medicare Advantage insurer provides a formulary that lists the drugs that they include in their plan, along with any additional details, such as whether there are any restrictions or what payment level the drug is under.
The formulary can change at times, and drugs that are removed will be replaced by a generic or other comparable therapy. Drugs that are not part of the formulary can also be requested to be covered through a process with the insurer.
I would definitely file for inclusion of the drug outside of the formulary, and at the same time determine which insurers in your state offer the drug in their formulary. Blue Cross Blue Shield of North Carolina has Oxycontin in their forumulary for both Medicare Part D (prescription) plan and their Medicare Advantage plan. You need to check the insurers in your state to determine which cover this medication, as well as which can best meet your needs.
Knowing that you have the option to ask for the drug and have it considered for coverage can be comforting. Knowing that other insurers in your state cover it can be a relief and used to encourage your insurer to cover it, whether in formulary or through a request.
Jonathan