Of course no drug is officially more addicting than another drug. It's all based on the user.
However, from what I've witnessed people struggle with quitting heroin more than anything I've ever seen. I've known people to quit opioids and they definitely have relapsed during the quitting process, but when it comes to heroin I only know people who have quit through long-term Suboxone use or methadone. They were not able to stop using heroin like I've known people to stop using oxycodone. I don't know anyone who has willingly quit heroin like I do with folks who have willingly quit oxycodone (without subs or methadone treatment).
Also, people seem to be able to hold onto oxycodone stashes and stagnate their use so they can avoid withdrawal. When it comes to heroin use, I've never known anyone be able to save it for a rainy day. They go through their entire H stash in a day and then go back into withdrawal. There's seemingly something more compulsive about heroin and the desire to redone over and over again--hence why I think it's more addictive.
I was just asking why heroin seems more addictive for most people, but obviously oxycodone is in itself extremely addictive too. I've also seen a plethora of people state that making the switch from oxycodone to heroin was their biggest life mistake. So I do think there's something way more captivating about heroin than oxycodone. It could just be that in general H is stronger than oxy (I've heard that oxycodone is about 1/3 as strong as heroin but that's super variable and a hard claim to back-up).
People have also reported than on oxy they had lives. Once they switched to heroin it's all they've wanted to do 24/7. I'm not trying to assert that anything I'm saying is true or that heroin is definitely more addictive than oxy. Just curious about the subject.