OTF is a gang acronym which stands for "Only Trey Folks" in reference to the Black Disciples gang. Trey is one the Black Disciple Nation's symbols, which is the roman numeral "III". FOLKS is the alliance, often referred to as a nation which the Black Disciples gang is a member of.
Lil Durk, a Black Disciples gang member and rapper from Chicago owns a rap group that goes by the same acronym, however, Lil Durk insists that the acronym of his rap group OTF stands for "Only The Family".