I suppose it depends on the relationship between two people. If the step mother really good enough to the kid and kid also accept her. He / she may call omma. Usually, Korean people call their good friends' mother "어머니/어머님". So if the relationshio is soso, they may call step mother "어머니". But usually, the relationship could be worse... in this case, they don't even "call" the title.

biological mother is "친모, 생모, 친어머니, 친엄마" and the appellation is "어머니, 엄마" stepmother is "계모, 의붓어머니, 새어머니, 새엄마" and the appellation is "어머니, 엄마 or 새어머니, 새엄마" depending on the intimacy. mother is "어머니, 엄마" and the appellation is "어머니, 엄마" mother-in-law is "시어머니,장모" and the appellation is "어머님, 장모님"

Thank you so much