OPC cards are bi-lingual, with both English and French writing. they also have ragged borders as a result of their being cut by wire instead of blade. The paper stock was usually whiter than Topps, which was often brown. although to say OPC was white is not really accurate...cream would be moreso. A lot of their appeal has to due with the fact that they were not available in the US so for long time collectors, pre-internet days, there was an allure of mystery about them, they were more exotic than Topps cards.
For the NHL, the OPC sets were usually double the size of the Topps set, and at least in one year, perhaps more, there was ONLY OPC, no Topps set, issued.
Finally, especially important when you are collecting the days when there was one set per year, it gives you another card to add to your collection, one that's slightly different.
Now that I'm collecting hockey, I also know there is a historical continuity aspect to them. The first OPC sets date to the 1930s, and are still being produced today- although there were breaks in there, and they don't make their own now, but are produced by Upper Deck...it's still O-Pee-Chee.
VERY slow trading due to health problems. Not transferrable so safe to trade with, just moving is painful and can't always access the cards.
Cardboard History My COMC
New Collection Website: Cardboard History Gallery (Still under construction)
Tips on how to make your scans look like the card does in hand (No more washed out, fuzzy scans!):