Why can't Momoe Okonogi see Yuuko?
As Yuuko explains, people see her how they believe she'll look, it could be that Okonogi  believes ghosts exist but can't be seen unless they're using a medium like with the doll Teiichi exorcised which would be backed up when the "evil spirit" disappeared. 

Given that since Teiichi to her is a spirit medium she is probably expecting Yuuko to be invisible and use Teiichi to speak to everyone which would explain why she can't even hear her.