Under the Open Source Software Development (OSSD) model, many renowned products have been developed that appear to be high quality. Advocates of the OSSD model claim that the achievements of high quality largely come from user testing and peer reviews. We assume that Quality Assurance (QA) methods under the OSSD differ from traditional approaches and examined mid to large sized Open Source (OS) projects to identify applied practices. We found the existence of common key practices, but often processes are more informally applied without clear definitions. The findings contribute to the development of a QA framework, which suggests QA processes and product quality targets. The model offers processes to assure that the product achieves its quality goals. We conclude that the framework is an important element, which requires the balanced interaction of human factors, management skills and a suitable environment to achieve software quality.