Nomenclature
The Oromo were formerly called Galla by non-Oromo Ethiopians, and one may encounter this name in older texts, but it is considered a pejorative term. Historically, some people among the northern Amhara community used the label "Galla" derogatorily to label Oromos as well as to label Shewan or southern Amharas who were mostly mixed with Oromo.
However, when Charles Tutschek, writing in the mid-19th century, researched the Oromo, "his informants, according to their published letters, used Galla as a term of self-reference."
During the 5 years of Italian rule over the whole Horn of Africa (a colony in Eritrea was set by Rome in 1870 and Ethiopia occupied in 1936), Italian geographers accurately mapped the population of their colony and eventually referred to the Oromos preferably as Gallas in all the official maps as well as in a guide-book still available nowadays called "Guida all Africa Orientale Italiana" ("A Guide-Book to Italian Eastern Africa"). The books stated the term Oromo was simply an alternative to Galla.
Often in the past, some Oromo communities used Galla to label themselves, as was exemplified by western Oromo leaders who established the "Western Galla Confederation" in the 1930s. The name has fallen into disfavor and is now considered to be pejorative, possibly because of a folk etymology for "Galla" (that it came from Qal la or "قال لا," pronounced similar to Gal la, Arabic for "he said no") that implies they refused Muhammad's offer to convert to Islam. In the Somali language, the word gaal means "non-Muslim" or "stranger", a possible reference to the Oromo and their old pagan religion.
Read more about this topic: Oromo People