darius is considered OP just because how he's usually in the spotlight (taking quadra and penta kills)
he's like the champ that grabs the most attention just by doing those quadras and pentas..that's why we have those who are "secretly OP" or "sleeper OP", they are really OP but they're just fading from common sight.
now, is he actually OP? I hardly think so, he's a cleanup crew most of the time, just dunking people with 700-900 true damage. seems OP? well, try taking down your enemies down to that 700-900 hp level by your own. you'll notice that if you build damage, you're ****ed and is dead within the duration of 1 polymorph. if you build tanky, your scaling abilities are laughable. his strength is only damage damage damage damage, but yet he needs to get close to do that and so is susceptible to kiting champs like janna, ezreal. other than that, a mediocre single target slow, but a good apprehend is borderline gamechanging if you can drag 3-4 champs and a cleanup ult designed to "kill secure".