The SAGE Dictionary of Qualitative Inquiry defines oral history as follows:
This is a method of gathering and preserving historical information recorded through interviews with participants in past events and ways of life. Oral history aims to gain a more complete or unique understanding of the past as experienced both individually and collectively by soliciting memories, reminiscences, and testimony from specific informants or respondents.
This short video from SAGE Research Methods provides an overview of oral history projects and how to integrate interview information with other source materials.
StoryCorp has a smartphone app that you can use for recording your oral history. Optionally, you may also archive your interview on their website (and at the Library of Congress). Make sure to get permission from your interviewee before doing this.
You an also export the audio file without publishing the story. Here are some helpful links.
There are also some more generic guides available. Note that many of them suggest you use an external microphone for better sound quality.