Tiers in general are not a direct representation of power or usability . Smogon tiers at their core are simply based upon usage rate . The reason Pokemon hit OU are because they ' re used often enough to hit that benchmark , which is typically because of the things they bring to the table in utility or damage or whatever . There are easily Pokemon not in OU that can be used effectively in OU , for example Amoonguss was tiered at around I believe RU last gen for a good bit , but Amoonguss is easily viable in OU because of its primary role of a Pivot . It ' s also a great counter to Keldeo because of its typing . Tangrowth is another great example , as it was tiered again at around RU for a majority of the gen , but as people started using it more and more because they liked how it played and did it ' s roles that it started moving up to OU usage rate .