Motivation - This research aims at investigating emerging roles and forms of participation fostering design-use mediation during the Open Source Software design process Research approach - We compare online interactions for a successful "pushed-by-users" design process with unsuccessful previous proposals. The methodology developed, articulate structural analyses of the discussions (organization of discussions, participation) to actions to the code and documentation made by participants to the project. We focus on the useroriented and the developer-oriented mailing-lists of the Python project. Findings/Design - We find that key-participants, the cross-participants, foster the design process and act as boundary spanners between the users and the developers' communities. Research limitations/Implications - These findings can be reinforced developing software to automate the structural analysis of discussions and actions to the code and documentation. Further analyses, supported by these tools, will be necessary to generalise our results. Originality/Value - The analysis of participation among the three interaction spaces of OSS design (discussion, documentation and implementation) is the main originality of this work compared to other OSS research that mainly analyse one or two spaces. Take away message - Beside the idealistic picture that users may intervene freely in the process, OSS design is boost and framed by some key-participants and specific rules and there can be barriers to users' participation