As Psycho stated above, it depends on what you mean by overdosing. Are most people getting high by going over the recommended dose for pain control? Yes. Are some people playing with fire and consuming near-fatal doses? Yes.
There's a few contributing factors to be considered.
Metabolic differences- Individuals metabolize drugs differently due to age, weight, medical conditions, etc. A young person with no comorbities may metabolize and excrete a drug faster than a 90-year old person. Also consider 90lb woman vs 350lb linebacker. Also, health conditions like kidney and liver disease may effect drug metabolism.
Individual tolerance- An 18 year-old has been prescribed percocet (5/325) for a broken ankle. He takes two (equating 10mg oxycodone) and it knocks him on his ass; he's slurring his speech and his eyes are half closed. Meanwhile, a 55-year old cancer patient is taking 30mg oxycodone every 4 hours for chronic pain and does not appear inebriated.
Drug Synergism- Combining drugs with alcohol, tobacco, potentiators, other drugs, etc. A 27-year old injects 8mg dilaudid and 30mg oxycodone by mouth without incident. The following day he takes several xanax bars and has a few drinks. He decides to add opiates in the mix since he's still wide awake; he repeats the same combination he took the night before. He dozes off on the couch and ultimately stops breathing.
So I suppose you could say most people seeking a high of some sort are over-dosing in a sense that the dosages we utilize exceed the recommended doses for the drugs intended purpose. Majority of us are not seeking a lethal overdose, but for some of us it can be a thin line. One can only hope that people do their best to educate themselves and if they're going to get high that they do so as safely as possible. Also it helps when friends/bystanders can recognize a potentially lethal OD and get the person the help they need. A close call and waking up in the hospital is better than not waking up at all.
And damn, I miss the days when I got a nice buzz off ONE percocet! :D Stay safe my friends!