Abstract: Background?During the last decades, the Open Access paradigm has become an important approach for publishing new scientific knowledge. From 2015 to 2020, the Trans-O-MIM research project was undertaken with the intention to identify and to explore solutions in transforming subscription-based journals into Open Access journals. Trans-O-MIM stands for strategies, models, and evaluation metrics for the goal-oriented, stepwise, sustainable, and fair transformation of established subscription-based scientific journals into Open-Access-based journals with Methods of Information in Medicine as an example. Objectives?To present an overview of the outcomes of the Trans-O-MIM research project as a whole and to share our major lessons learned. Methods?As an approach for transforming journals, a Tandem Model has been proposed and implemented for Methods of Information in Medicine. For developing a metric to observe and assess journal transformations, scenario analysis has been used. A qualitative and a two-tier quantitative study on drivers and obstacles of Open Access publishing for medical informatics researchers was designed and conducted. A project setup with a research team, a steering committee, and an international advisory board was established. Major international medical informatics events have been used for reporting and for receiving feedback. Results?Based on the Tandem Model, the journal Methods of Information in Medicine has been transformed into a journal where, in addition to its subscription-based track, from 2017 onwards a Gold Open Access track has been successfully added. An evaluation metric, composed of 5 scenarios and 65 parameters, has been developed, which can assist respective decision makers in assessing such transformations. The studies on drivers and obstacles of Open Access publishing showed that, while most researchers support the idea of making scientific knowledge freely accessible to everyone, they are hesitant about actually living this practice by choosing Open Access journals to publish their own work. Article-processing charges and quality issues are perceived as the main obstacles in this respect, revealing a two-sided evaluation of Open Access models, reflecting the different viewpoints of researchers as authors or readers. Especially researchers from low-income countries benefit from a barrier-free communication mainly in their role as readers and much less in their role as authors of scientific information. This became also evident at the institutional level, as Open Access policies or financial support through funding bodies are most prevalent in Europe and North America. Conclusions?With Trans-O-MIM, an international research project was performed. An existing journal has been transformed. In addition, with the support of the International Medical Informatics Association, as well as of the European Federation for Medical Informatics and of the German Association for Medical Informatics, Biometry and Epidemiology as European and German medical informatics organizations, we did run an international experiment on Open Access incentives. Both together are, as far as the authors know, unique. We therefore expect that this research could add new knowledge on Open Access transformation.