Nothing, oxytocin is almost totally inactive in normal people though some with autism spectrum disorders do get a mild effect. It's got poor dose kinetics anyway.
Even if there was an effect it would be drowned out by the natural release of serotonin and other peptides like oxytocin.
Sekio just stop it already.
Have you taken oxytocin yourself? I didn't think so. You have no clue.
I posted a few years ago, on this very website, my own trials of oxytocin administered nasally while on MDMA. I think it does have an effect when you are sober and it is slightly different on MDMA, probably due to a unique synergism between what the drugs do.
That is not to say it is worth a lot to go after, but it is there. The biggest problem with oxytocin is that it also activates vasopressin receptors, which causes diuresis (ie, you have trouble peeing). This can be particularly problematic because MDMA already does this, so you may not pee for a quite a while after doing both.
This is assuming what you purchase is good quality oxytocin though. I wouldn't mess with 95% of what is probably available for sale. There is probably one or two reputable peptide suppliers that will give you something worth using. Please don't buy just anything from a cheap sale website. If they are advertising it as an antidepressant or whatever, don't buy from them. Only buy from a site that sells other peptides, and is clearly not in it just for oxytocin sales but has an actual peptide business going.
I also took it sober and notice effects on memory retention, particularly related to facial recall (as has been widely published) and the same diuresis. It also made me tired.
You can get a good head rush from it when done in the middle of an MDMA experience, but like I said, the bad effects outweigh the good really. Search for my old thread on it in the experiences forum, if it wasn't deleted... it was at least 3 years ago.