First Mentioned: Hvs.tCF 208
Magic that has been ruled OP by the Magic Kingdom includes:
Magic that is has not officially been ruled OP but is often considered to be includes:
- Awakened towers
- Learning golems
- Manipulating bedrock
- Unlimited juice
- Carnymancy/Thinkamancy caster links
Given the usage of the term in Hvs.tCF 208, the Arkentools had not previously been delcared OP, suggesting that the criteria for something becoming OP is that it must be shown to be game breaking, such as Decryption, rather than just having the potential to be game breaking.
Another interpretation is that the OP designation is primarily political: the Great Minds do not care about the actual imbalance of power among other sides and factions so long as they retain their secret control over the Magic Kingdom. Their goals seem to include the preservation of Thinkamancy secrets, preservation of the Magic Kingdom, and the defeat of Charlie. "While it is true that Parson Gotti will be hampered by the loss of his Croakamancer and some major units, Charlie will be much more grievously restricted. This is viewed as a significant net gain against Charlescomm." Erf-b3-p227
The shunning and forbidding of OP magics implies that it is not enforced by Signamancy. Most likely due to it being impossible to impose a Contract on those that did not Sign it automagically.
It could be that the secretive nature of the Great Minds is out of fear of having their discipline declared OP.
Real World References
OP is an acronym for overpowered, a gamer term for imbalanced game mechanics.