An oral heirloom is an interview captured by digital (later, we'll explain why) audio or video equipment. The interview itself can be formal -- accounts of historically significant events prompted by well-thought-out questions -- or imprecise, such as simply reminiscing about past times.
Keep in mind that these tales aren't meant for mass consumption; it should be a one-on-one conversation. So public speeches are out -- and forget filming political debates for preservation. Oral heirlooms aren't a secret affair, either. No wire-tapping or personal diaries.
Instead, make it a dialogue between an interviewer and a subject that is recorded. This is an important distinction between an oral heirloom and one that occurs, say, when you turn your video camera into a Blair Witch-style documentary. These interviews are deliberate and planned; they can be transcribed, summarized and archived. This means the interview lives on; it can be researched, used for publication or even become part of a museum exhibition.