This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Kamen Rider OOO
Every single time Eiji has a flashback to the young girl he made friends with, dying from the missile attacks and Eiji's breakdown as a result.
The massive Yummy from the second episode, where it takes on a bug like appearance, and the upper portion of the body is grasping hands.
Professor Maki's doll. Know what? Maki in general.
Speaking of his doll, watch it closely whenever it appears, and take note on what happens when they change cameras. Haven't caught it yet? The doll can see you...
Maki's panic attacks whenever his doll is "hurt". It's the only moment he shows an emotion, like EVER! And it is not the best.
Episode 10. Not only is the bomber extremely unsettling, especially when you consider how close they all were to exploding (Eiji managed to stop with one second to spare), but there's what Maki does at the lab. He knows one of Mezool's nests are there and they're ready to hatch. So he decides to put the whole place on lockdown, with all the scientists inside and the Yummy's starting to hatch, just to see what would happen.