Actually all the Showa era Kamen Riders are technically set in the same universe. It's only the Heisei riders that start with clean slates (except for Kuuga -> Agito). And of course, W and OOO.
^ well OOO is kinda related to W in a way tho, with the Kougami Foundation thingy.
Actually, the link in common is Foundation X, because towards the end of W, one of the agents of Foundation X pulls up info on both the Gaia Memory "investment" and the "Cell Medal" project that was basically the basis of OOO. Unless Kougami Foundation = Foundation X, which I highly doubt... It's more likely that Foundation X is backing Kougami Foundation in the Cell/Core Medal research, the same way they backed Museum in W for Gaia Memory research. They're kind of like Evil Investment Inc. Lol.
Also, for linked riders...there's always Decade for some serious continuity breaking.
After all, he's the Kamen Rider who's always just "passing through".