The talk presents a concept of Open Science-related training developed with the aim of facilitating the adoption of institutional repositories in a specific local context (Serbia). This context is marked by a considerable delay in infrastructure development, the lack of institutionalized training on Open Science, a knowledge gap, and insufficient library staff. Although institutionalized training is indispensable in the long-term, this example shows that a combination of bottom-up approaches and highly customized and informal training can increase the Open Science capacity of researchers, librarians and even institutions, which is highly relevant in the context of building competence and capabilities for EOSC. In these terms, it may be instructive for other environments with poor formal training on Open Science. The adoption of the Open Science Platform (the national OS policy) in 2018, spurred the development of institutional repositories in Serbia and the leading role in this process has been taken by the University of Belgrade Computer Centre (RCUB). Before the adoption of the OS Platform, training on OS was mainly provided through international projects. The official Library and Information Science curriculum, Librarian Licensure Examination programme, and professional development courses for librarians and training programmes offered by university libraries did not address OS-related skills, due to which there was a knowledge gap in the library and research communities, and this threatened the adoption of the developing infrastructure. To overcome this, the University of Belgrade Computer Centre decided to include training (for repository managers and end users), along with software, hosting, and technical support, in the service package offered to institutions. A dedicated user support team responsible for designing and implementing training was established. The training programme covers a range of topics beyond repository features and workflows: Open Access policies, FAIR principles, metadata standards, copyright, self-archiving policies, altmetrics, dissemination through metadata harvesting, discovery platforms (OpenAIRE, BASE, CORE), using institutional repositories in the context of Research Data Management and cultural heritage. Training formats include predefined lectures and webinars, but also highly customized sessions and informal consultations. As a result, dozens of repository managers (mainly librarians) and hundreds of end users (researchers) have been trained so far. A number of trained repository managers have already started organizing training on various OS topics at their institutions. Together with the user support team, they form a strong network enabling dynamic information exchange. At the same time, there is a growing interest among researchers for additional training on particular OS topics (RDM, copyright, integration of the repository in various institutional workflows). Also, continuous and flexible support encourages content diversity in repositories. The concept of training developed by the RCUB user support team has so far proven to be efficient in mitigating the lack of institutionalized training. However, in order to provide full support for all aspects of Open Science (esp. RDM and citizen science), it will be necessary to establish institutionalized training.