Leona is in no way shape or form OP.
She is EXTREMELY reliable if you have a team that follows up, because you can root a target for about 5 seconds with her full combo or you can fuck up 3 individual targets with her full combo, more than that if you hit your ult dead on more than one.
However since soloQ is basically full of tards that have the reaction time of a potato in a slow cooker, she is incredibly risky as you might engage a fight and lock down the enemy APC and ADC, but by the time your team reacts you are either dead, or they react by focusing the frontline and you get melted by Vayne's bullshit.
TL;DR
Incredibly strong with a good team? Definitely.
Very reliable and reasonably easy to use? Absolutely.
OP? Not in the slightest.
A Fiddle has more chances to run away from getting caught out than a Leona does.