O.D. is typically soft clipping and lower levels of distortion.
Distortion is typically hard clipping and high levels of distortion.
Using O.D. pedals for distortion is fairly useless, for me. I use them as a signal boost and to boost e.q. frequencies I want to emphasize. People running the gain/drive high on an O.D. are missing the point, IMHO. But then again, to each his own.
Also, on fuzz and distortion, I think people miss out on the signal boost aspect. Jimi Hendrix's fuzz tone was AWESOME because he ran the Volume on 10, thus overdriving the input of the amp. If you run a fuzz at unity, you are missing at least half of the magick.
Then there is old school pedals like my Colorsound Overdriver clone or a Rat Distortion, which are using fuzz like circuits to achieve distortion tones.
Some of my absolute BEST tones come from running pedal volumes on 10 and using the guitar volume to control the drive. This has been the best way to achieve "Cranked" tones and amp interactions at low volume for me. Amp volume set clean and low, pedal dimed to drive the hell out of the input, guitar volume barely cracked open for rhythm, and just slightly higher for singing leads. You know you've got it right if the guitar volume on "10" is all but unplayable. It mimics playing a plexi on "10" in that you have to ride the guitar volume if you expect to have any control, even at low overall volume levels.
The Colorsound circuits I own seem to do this better than anything else I have tried. The Overdriver and one knob fuzz send huge doses of signal to the amp.