I'm reading privacy documents and this words came up, I have no idea what it exactly means.
"Independent oversight" means that the activities are done under the principle of segregation of duties and by parties that do not have any reporting line to key stakeholders of the data or the outcome of the data analysis, to avoid the so-called "conflict of interest".
This is why you have consulting or auditing companies providing you with data and audit reports, or overseeing the process, to avoid deliberate tinkering with the data / sampling procedures.
In the manufacturing industries, this is why Quality Control and Quality Assurance should never report to Operations. In the stock market, this is why companies like PwC, KPMG, EY and Deloitte are contracted to provide yearly assessments of a company's finances.