O kawaii koto
The Japanese version of ‘how cute’. The character Kaguya Shinomiya uses this a lot in the manga called Kaguya Wants to be Confessed to, otherwise known as Kaguya-Sama: Love is War.

It is Shirogane Miyuki’s (another character) least favorite saying of hers.
Kaguya: O kawaii koto.
Prez/Shirogane: AHHHHHHHHH