As others have said, we aren't here to help you get high and we can't tell you how much to take in order to get high. I'll leave this thread open for now but please stick to the Basic Drug Discussion Guidelines (which can be found in my signature).
Each effect of a specific opioid, such as euphoria or sedation, is independent from the other effects and the levels do not necessarily correlate. Even one's tolerance to each effect progresses at a different rate. It's not like one can determine the dose needed to feel sleepy and then somehow extrapolate the dose to feel euphoria. Sensitivity to specific opioids depends on your metabolism and your unique brain. Opioids are metabolized by different enzymes, and each individual has different levels of these enzymes, which can affect how strong a drug will be for them and what effects they will get from it. There are likely other mechanisms at play as well, but it is a complex thing and individual differences in euphoria haven't really been studied because euphoria is not generally considered the desired effect of opioids in medicine - analgesia is. Some people may even find more euphoria, stimulation, or sedation in lower doses with certain opioids as opposed to higher doses. The only safe thing to do is start low and gradually titrate your dose up if needed, while being careful to stay within the safe dosing range.
I haven't heard anything about opioids in general "not being fun" for schizophrenics, and there are plenty of schizophrenic heroin addicts. Perhaps it depends on the type of opioid? Or maybe schizophrenics who use opioids do not do so because they produce euphoria but because they reduce their symptoms? I have seen a few anecdotal reports of some schizophrenics saying they got negative effects (like exacerbation of their symptoms) from codeine or oxycodone. But there have been studies on opioids (morphine, methadone, heroin, hydromorphone, etc) helping schizophrenia. There have also been some studies that schizophrenics tend to have an imbalance in natural opioids. It is also strange because I have also read studies where naloxone (which blocks opioids and is believed to block the body's natural opioids) was found to reduce hallucinations etc in certain schizophrenics. It is still not understood how pharmaceutical opioids affect schizophrenia, but the research and anecdotal evidence seems to show that they usually tend to help. I'm certainly not suggesting you try other/stronger opioids though, for obvious reasons.
I would be inclined to think that the particular opioids you have tried just don't work well for you. It is quite common for people - believed to be about an average of 1 in 10 - to not get much out of codeine, hydrocodone, tramadol or oxycodone.
I think what Ho-Chi-Minh meant was that the "euphoria" caused by opioids can be quite subtle and people often don't appreciate it or notice it the first few times they try opioids.
I think it's impossible for us to determine the exact mechanism(s) behind why you didn't feel anything from codeine or oxycodone and I doubt there is something that you could do or take to combat it anyway. Maybe take it as a blessing in disguise that you didn't like them?
