Depends how you define OP. I don't mean to get all technical but we'll assume that means, fundamentally, a comparable value that identifies and reflects a particular champion's "strength" on a relative scale. This indicator would obviously be... DPS (damage per second). There are probably videos or even forums where league enthusiasts have combined mathematics and gaming to provide a numerically based perspective on character strengths and thus ranked them accordingly to how "OP" they are.
This "DPS" statistic is predicated on the champion class itself (mages/AP casters and AD/Marksman should have a higher DPS than Support classes) and items that are purchased throughout the game. Therefore this value is bound to change during the course of gameplay and even among different builds on a particular champion. Hence, it isn't a perfectly accurate scale or indicator to assess character performance OVERALL (which is probably what reflects an OP champion, OVERALL better than the others). An ideal system would incorporate compensations to balance the "unfairness" or "differences" between the classes, such as damage output. Their supportive abilities/passives would also be factored into account and perhaps given a score or rating which is modulated or attenuated specifically by a pre-set factor to assign each skill with an appropriate weighting. So skills that involve healing (or have 0 damage output like polymorphs or certain stuns) wouldn't be disqualified but instead counted in such a way that its effect on the game would have an appropriate percentage of influence on the champions "overall score".
There's a lot more I wanted to go into detail about but unfortunately I'm a little pressed for time, that's just a general summary of what I think.
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