Which phrase is correct? "Oh, God forbid." or "Oh, God forbids."
TV show context:
Father: The men and women of this hospital live in separate wards for good reason. In the past there have been incidents of violence and indecent behavior.
Daughter: Oh, God forbid!

Why is the subject-verb agreement not followed?
Thanks for your answer.

Question

Which phrase is correct? "Oh, God forbid." or "Oh, God forbids."
TV show context:
Father: The men and women of this hospital live in separate wards for good reason. In the past there have been incidents of violence and indecent behavior. 
Daughter: Oh, God forbid!

Why is the subject-verb agreement not followed? 
Thanks for your answer.

Answer

This is a shortened form of "May God forbid [this]."

Here is a related discussion of the subjunctive in phrases such as "God bless you."

https://www.italki.com/question/472405

Answer

"Oh, God forbid!"
Because it is in imperative form.
Same for "God bless you".

Answer

God forbid
https://dictionary.cambridge.org/dictionary/english/god-forbid
>>subject-verb agreement not followed? 
It's an idiom aka tradition.