Im really curious why okaya was so obsessed with her son-

I think it's because he was the one person who could defy okaya like. Ok like all that bullying and tormenting all that and her son never broke, never gave up, and never backed down, he kept thriving living and disobeying. Okaya wanted to see him break to him it was basically a game. That's why when he died okaya said he won because he escaped and it wasn't because of suicide. I think

When a unhappy beaing find its new toy to play with that defy his own rule thing will get twisted but karma will follow ahead.