I'm not familiar with Den-O at all, but it appears time travel is a major factor in that series. that always leads to trouble! this is just a preview, as I haven't watched the entire movie.
when the OOOs kids show up and travel back to 1971, this happens. Kamen Rider OOO has something to do with collecting discs (gotta catch 'em all?) and drops one in the past. that leads to all sorts of complications in 2011.
so back we go to 1971 and we get all sorts of Riders showing up eventually to right the wrongs that have been happened.
this is a cool shot of our heroes about to take action. that is a lot of Riders about to wipe out a poor measly Shocker squadron. and probably some kaijin as well.
here's what I was looking for! however, it's a very short cameo, but very cool. I'd love to see a real movie featuring all four of these heroes. that would be really cool.