If we understand validation in the sense of positivism, i.e., truth validity, then there is no room for any 'meaningful' discussion of validity in an ethnographic study. Instead of validation, what an ethnographer must be looking for are contextualised meanings of the phenomenon being studied. Even if the oral history(ies) is not true or deviates from the commonly known account(s), it still holds relevance in an ethnographic study as it throws light on the underlying explicit or implicit cultural or psychological factors for such variation. I think validation in an ethnographic study is a harmony between the oral account(s) and the culturally nurtured meanings of the account.
Well, Try Triangulation Methods, use multiple sources such as interviews, observations, and data documents and other forms of data to verify and confirm details. Using different research methods such as interviews, participant observation, and document analysis to gather information from different perspectives. Then Crosscheck, compare information from different people who may have experienced or observed the same event Consistent details across multiple accounts make oral traditions more reliable. Go with Comparative Analysis, examining oral history in relation to other historical records, official documents, or outside sources that can provide context or confirm details.
All of the above suggestions are good. I've found that the known archaeology about the group is also useful for validating oral history. For example, the Hopi of northern Arizona have an origin story about coming from Pala'kwabi, "The Red Land of the South," Many material items mentioned in the tale -- macaws, pottery, The Plumed Serpent, etc. -- in Hopi culture arrived there through ancient trade from Mesoamerica. Thus this part of Hopi oral history is validated, but not in exactly in the way they intended: it's the origin of many important aspects of Hopi culture, but perhaps not the origin of the Hopi themselves.
What I love about your question, as an ethnographer, is that it indicates that what is being taught as ethnography today has lost the full meaning of ethnography, since if you include classic ethnography techniques along with methods like oral history that are the focus today (to add what George has, precisely, called "triangulation" of multiple data sources) you will already have the confirmation that you need. I will point you to some examples and also help you to recognize the real richness of our field of ethnography as it was originally designed: to look holistically (as a whole, with multiple confirmation in a unified way) at "ethnic groups" in their territories/landscapes (geographic, ecological, and relational niches) using full "participant observation" of everything that can be sensed, including quantitative methods and multiple disciplines, and not just oral histories or written materials. There was a good reason why Anthropology was once four fields, to include archaeology and linguistic anthropology (and sometimes physical anthropology, if you are looking for evidence of relations and movements of different peoples); because these subfields add techniques that include the historical record to confirm them. Today, genetics and linguistics offer increasing information that confirms historical interactions and movements, in addition to archaeology, that always has.
Although landscapes are being destroyed today with globalization and "modernization", in many places, including India, where you are today, you can still see historical evidence from multiple periods and cultures eveywhere around you if you know how to look; in eveything from cemeteries to structures (especially the underlying foundations -- I have found 3rd century influences from India in the Buddhist towers in Laos, for example, which confirms oral histories of something that happened almost 2,000 years ago) to types of plants/trees (evidence of different trees brought from India to Southeast Asia, for example) to just patterns on the ground or waterways that indicate changes introduced or naturally occurring at different times.
Take a look at my articles, "A New Approach to Heritage Tourism in Southeast Asia ...", "Taking History Back to the People", and "A New Heritage Classification Scheme ..." that are on ResearchGate for examples of this kind of ethnographic methodology that you can use.
Mr Ashoke. Greetings. we Indians are one of the oldest civilizations, and we have preserved our Hindu mythological texts, history, and lifestyle practices for millennia through oral history. The scriptures on palm leaves went to the scholars. However, the rural and common man got it through the verbal transformation of information as storytelling. We historically used Tholu bommalu and Cherial-like Art forms and made dolls and idols to play in puppet shows. Stage dramatics in rural festivals, Folklore during all family functions (For example. when a child is born, he was praised and blessed to grow up like King 1, King 2, etc.), The temple sculptures, ancient wall arts, our heritage home decors and dressing culture, naming the children everything revolved around them and kept preserved over generations. If you study this evidence ethnographically, in short, if you study the traditional arts and crafts and their elements, which are currently present, and study a few of the cultural practices, they will validate how orally the history has been preserved using these cultural tools.