Hey Nick and welcome to Bluelight! We are very happy to have you as part of the community! This is actually what I consider a fairly easy question. Firstly, in the future, you can use what is called an Opioid Conversion Calculator to do perform a pretty accurate dose conversion yourself. You don't even need us, which is great, because you can now arrive at a safe dosage estimate without having to wait around for one of us, but for now, I shall help
So, for most folks Codeine (3-Methylmorphine) is demethylated into Morphine at a rate of approximately 10%. This means, and again, this is for most people, that if you take 10mg of Codeine, you will end up with an equivalent dose of Morphine equaling approximately 1mg. In your case, we can now determine with ease that your 300mg of Codeine will metabolize in vivo to approximately 30mg Morphine.
Now that we have determined your Morphine Milligram Equivalent (MME), it will be easier for you to convert to Oxycodone. Every 1.5mg of Morphine is equal to approximately 1mg Oxycodone. This means that Morphine is 2/3 (66%) the potency of Oxycodone by the oral route. If you want to discuss different routes of administration, we can do that, but you're not likely to be able to process your Oxycodone into anything usable by anything other than the oral route.
So, now we know that if 30mg was your MME dosage, the equivalent dosage of Oxycodone would be approximately 20mg. This will be your starting point, my friend. You can use this as a benchmark, but as I always say, everybody is different so tread lightly. Not too complicated right? Here is the link for my go-to conversion calculator, although after a decade of junkiedom, I admittedly know most by heart
