Hey!
I born and grew up in Oromia region. Which mean the area that working language is Oromifa. To your surprise, I don't know even greetings. My message here is, it is not mandatory to know Afan Oromo unless you want to gain more knowledge. The people is nice and everybody can understand more or less. Especially at the city side, Both Amharic and Oromo languages will be used, but Mostly Amharic is used for everyone. If you are thinking to be at the countryside of specifically Oromia region it will be better if you start learning, still not mandatory. If you are planning to live another region Oromo language never be used. In short, Amharic is the basic language to survive in Ethiopia, Since it is the official language of the country. Ethiopia is collection of 82 languages.