In a resource based game, power is tied to cost, thus the meaning of OP is equivalent to being under-costed. Every card is balanced against its cost. Every effect has a resource cost it is valid at.
With each OP card or each autoinclude card, the pool of legitimate choices shrinks, creativity decreases and the "deckbuilding" element of a TCG decreases as the decks increasingly build themselves. If costs were raised to a point where the current auto-include cards where being played at less than 3 copies for those factions, then this would indicate the game has become more balanced (Wealthy Noble is a good examples of a card that is 100% auto-include - the question is not "Should I include it?" But rather, "Can I justify not including it?").
Normally power is tied to a cards cost, but i still think, that a 100 cost "you instantly win" character would be a bit too strong with certain reanimator strategies^^.
Personally, i think a single card is OP, when the card adds too much to an existing type of deck and defines a whole meta which requires you to play very specific counter strategies to stop it. Examples include Pre-nerf Karani and breeders (in combination with trickster monkey) pre rise. Normally a deck can be Overpowered, if it either forces counterstrategies which are completely useless against most other decks to get a decent winrate against it or it is just has too high winrates against any deck. (Clearly those things are just impressions and it's impossible to measure). A single card can only be Overpowered as a part of an overpowered Deck, else the card is just strong but no problem.