Overpowered, or OP, is a term used in gaming to refer to something which is considered to have an unfair and unbalanced advantage over alternatives. In Hearthstone, it may be used seriously to question the design and balance of certain cards or decks, or simply to assert extreme power and effectiveness, and may be used as a complaint, a criticism, or a compliment.
The value and power of any card or deck is strongly influenced by the current meta. As a result, perceptions of being OP often come and go over time. Cards that are consistently OP for longer periods may be nerfed to help maintain a healthy meta. Cards originally in the Classic card set may be moved to the Hall of Fame set instead, removing them from Standard format without nerfing them.
One way of identifying overpowered cards is by comparing them to other cards of the same mana cost and seeing if they tend to be a dominant choice when a deck needs cards of that power level. For instance, Ragnaros the Firelord tended to be the default choice when decks needed an 8-cost card, pressuring developers to make new 8-mana cards at least as generally good as Ragnaros rather than focusing on deck synergies and a wider variety of mechanics. This eventually necessitated its move to the Hall of Fame.