How do you decide a champion is OP and needs fixed. Surely this goes beyond just win rates?

Totally and even the win rates we look at include multiple skill levels and across all regions. We also look at win rates for players who are newer to that champion and those who have played 100s of games. In addition to win rates we look at pick, ban and play rates, champion diversity overall, our recent patch notes for that champion, other changes going on that might affect balance (e.g. a bug fix or new item) and a bunch of stuff I am forgetting.
Even then, the data don't tell us when to buff or nerf. The data indicate potential problems that we discuss and decide based on experience, intuition and analysis if we should make a change.
Ideally we also have plenty of time to playtest the changes, but it takes a lot of a game to notice subtle shifts in trends and sometimes we want to react quickly.

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