The Oral History Association (OHA) defines oral history as "a field of study and a method of gathering, preserving and interpreting the voices and memories of people, communities, and participants in past events."
According to celebrated oral historian Alessandro Portelli, "Oral history changes the wiring of history much as the modern novel transformed the writing of literary fiction: the most important change is that the narrator is now pulled into the narrative and becomes part of the story."
The Oral History Association is a professional organization for those engaged in oral history practice. OHA has wonderful resources for beginners and experts alike.
Although oral history research is often exempt, check with your institution's IRB before beginning any research involving human subjects or participants.