>I couldn't focus (or really think clearly) after taking a dose

This might sound dumb, but I think part of it might depend on your experience with recreational drug use. The effects of Oxy are...off-putting if you don't know what to expect, and I'd agree that it might mess with your "focus". But, personally, I've found altered mental states to increase focus, once you can "harness" it.

Note: I'm not advocating experimenting with OxyContin, as it has serious physically addictive properties.

I was doing web development at the time, and I too found it hard to focus, especially on deeper problems - e.g. visualising deep stack traces or class hierarchies. I was able function, but it definitely required more conscious effort

Funnily enough I was trying to pick up React with my time off but I just couldn't make it do the thing. Next day, off narcotics, it was a breeze lol.