That's wild speculation. People who can reliably obtain pure drugs and properly dose their drugs are far less likely to die from them, obviously. Currently, most drug users cannot reliably obtain pure drugs (virtually all street drugs are contaminated) and hence cannot dose with complete accuracy.
That's true, but even when Oxycodone was easy to get a legal prescription for from any pill mill doctor, there were still hundreds of thousands of people dying from overdoses from their professionally-made, exact dosings.
The wild west going on with fentanyl is certainly worse, but I don't think legalizing what is likely the most addictive drug on the planet would improve things. Even if you cut down on contamination and improper doses, just the sheer increase in users would cause a ton of deaths.
A big part of the problem is that people who want heroin or oxycodone, in consistent doses, are getting unpredictable doses of fentanyl. Despite addiction to heroin or oxycodone not being great, it'd be a lot better if those people got what they wanted, rather than what they're getting.
