[Matrixbeast]

I'm voting for OOO.
After a little deliberation, I determined that Decade's various Rider forms can be countered by OOO's forms.
While only OOO's combos get the additional powers, OOO's versatility with the medals pretty much score him the victory. While OOO lacks a real projectile (Aside from TaJaDol's fireballs, he doesn't have much in terms of distance fighting, since the Kuwagata's lightning isn't concentrated and just sprays electricity around him, Unagi's whips can only reach so far, the MedaGabryu's gun/cannon mode and Gorilla's arm shot is only really used as a finisher, and the only thing else he has is the Tora Ridevendor's medal gun thing), various combos pretty much allow him to close any distance Decade could try to put between them.
Clock up and Faiz Accel are countered by LaToraTah's super speed, and Illusion is countered by GataKiriBa's cloning.

Close range, Decade could get an advantage using strong and agile Riders, like Kuuga, but I don't think he could get in any game changing blows.
I think this would be a close match, but Eiji is just fighting for more (And Eiji's desire directly fuels the medals' strength).

NOTE: This is all under the assumption that card uses and medal swaps are all free actions, meaning that Decade wouldn't slap the medals out of OOO's hands and OOO wouldn't kick the cards out of Decade's, etc.