Transmission of STIs and STDs

I understand oral is a way STIs can be transmitted. But I'm also a little confused.

Scenerio 1: female is carrier of an sti and is recieving oral...the person giving her oral would then catch the sti as well

But if in Scenerio 2: female is not carrier of an sti and is receiving oral from person who DOES have an sti...would the person eating her out be able to transmit the sti to her? If they're the one giving head?