Early on, (for oxycontin), you tend to get really jovial and mellow, and feel more motivated. Your weiner doesn't work, will stand at half mast, and it's almost impossible to orgasm. You enjoy doing stuff you normally wouldn't, like going to work, and are generally in a good mood when high. You get very constipated and if you're like me, would be lucky to poop once every 10 days. It becomes hard to urinate as well if you're a guy.
After the addiction starts to spread, you become increasingly depressed and sick without them. You get hot flashes, cold sweats, aches and pains, and remember that pooping every 10 days? It tends to come out as you're in withdrawl, so as you're sitting on the toilet freezing cold, shivering and greasy, you'll be anally raped from the inside out since by then, the crap that has been building up has lost most of its moisture and is rock solid and about as thick as a golf ball. I've probably broken the world record for the longest connected turd at the height of my addiction(although I switched to heroin by then). You'll lose weight, and the drugs will only make you feel some semblance of normalcy. Feeling normal becomes your new high. Eventually, there's a good chance that you'll switch to heroin too, and then it's off to the races.
Please don't try it if you're considering it. You won't notice the addiction taking hold until it's too late, and any willpower you may think you have erodes, along with your pride. I grew up anti-drugs and alcohol. I started smoking pot, then after a few years became extremely anxious and depressed from it. When I first tried opiates, it was like a piece of me that was missing my entire life was restored. I had self confidence, high self esteem and was happy. For every hour of happiness I had, I went through a week of absolute gut wrenching agony. It ruined my life over the course of 3 long years, and 6 years clean, I'm still picking up the pieces.