There are multiple definitions of OP. Typically OP is used more in reference of something that wins because it's too powerful to lose. At least in the older sense. The term gets thrown around a lot these days.
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What I'm saying is...
If average players don't make the Thunderer OP, then only good players do.
If good players are OP in the Thunderer, then she SHOULD outperform other choices the players could make. Kremlin, Bourgogne, Ohio, and others. However, her Strengths and Weaknesses are actually in balance enough to make a veteran player stop and consider the options... because the other ships might be the better tool for the job.
In this sense, I'm pointing out the "OP" is easy to say, but it's shown in actual practice that even unicum players see other "weaker" ships as strong or stronger then Thudnerer given context.