For many years when I was a Kid I would hear about "Oh Peachy" cards made in Canada. I think I would sometimes see OPC listed in price guides or info about the trading card hobby and wonder what it was. I forget how long it was before I learned and noticed that the Canadian brand O Pee Chee was a sort of sister company to Topps. Every year for each sport they produce a very similar product with some variations usually just in the info on the back has been translated into French (apparently from 1970 on they have had to include The French) . Sometimes the front designs would be slightly different but usually the only way to tell the difference was if there was French on the card, or to look at the copyright line (which in the early days for Topps was just T.C.G. with some other nonsense acronyms). If your card was off-center enough you didn't even have the copyright line on the back. If you get your cards directly from packs then there is no problem you know which brand you were getting, but if you got your cards from a shop, or traded with someone you might not know for sure.
Some years I think the OPC set was slightly smaller, so some of the numbering would differ, but I think they eventually went completely parallel to Topps.
Well anyway I've had a few OPC cards, even some non-sport versions. I've got some OPC cards from the Movie Grease mixed in with my hand built Topps set. I think there are 5 (maybe 6) cards that I only have the OPC version. Well usually I don't have both versions and can only tell some of the differences if I have other cards from the same set.
Recently from Ebay I got the OPC version of a Hockey Card that I had gotten the Topps version a while back. From the 1974-75 Season Card # 256 (both sets) the Wasington Capitals Logo Card. This was the inaguaral season for the Caps.
Notice the front of the card, there is virtually no difference. The colors on the OPC card are darker and richer, but that isn't necessarily normal.
1974-75 O PEE CHEE Hockey #256
Now the back is a different story (first my Topps version is off-center enough that the Topps copyright line is not on the card). The OPC version uses a whiter card stock and they seem to use a lighter brown color for the background color. Notice also the subtitle line "Draft Sellections June 1974" appears in English and French. If my Topps card had the copyright line, and the OPC version didn't have the French that would be the only way to tell the two apart.
It seems that in the world of Hockey card collecting most collectors prefer the O-Pee-Chee over Topps. Weird I've always been the opposite. I used to think the OPC cards were inferior, but eventually learned the only real difference was different card stock. It's weird some years I think the Topps version is better other years I think the O-Pee-Chee.