I've never used Painter 8. Painter 7 I found really difficult to use and gave up on it (plus it needed a lot more RAM than my 256MB could give it). I like OC because OC1 was a single self running executable that was less than 1 MB. And it's simplicity to use was great, despite it's lack of functionality. Plus it was free and didn't hog RAM like Painter and Photoshop did. When they released OC2, there was slightly increased functionality in my opinion. They finally included true transparency in layers instead of just Multiply-blended layering. There were also more tools like selection tools and some basic filters thrown in. They dropped the networking capability (or if they didn't I can't find it!) so you can't have PaintChat-like sessions with friends anymore. OC3 came along and it's got a lot of cool things like toning, more tools, tons more functionality. I honestly haven't used OC3 to make a serious image yet so I can't say how it is, but I know you can open OC1 or OC2 files and it'll keep the layers correctly. You can also open Photoshop .psd files I believe.

Painter is definitely the more professional one and can simulate brush strokes and textures far better. If you've worked with real paints before, you will like Painter tons. I personally don't paint with real media so I don't know what all the Painter brushes do (there's a steep learning curve in my opinion) but my Painter 7 came loaded with a shitload of tutorials that you can use to learn.