In this spreadsheet, the most granular category is Opoid Pain Relievers: https://www.drugabuse.gov/sites/default/files/overdose_data1...
Oxycontin's market share seems to be around 20%: http://www.jefferies.com/CMSFiles/Jefferies.com/files/Confer...
I don't think drug companies are held to account on whether doctors prescribe illegally...however they do get criticized/prosecuted for how they market their drugs: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2622774/#__ffn_s...