When newcomers try to join an open source soft-ware (OSS) project, they face many barriers that hinder their first contribution, leading in many cases to their dropping out. Many projects leverage the contribution of outsiders, and the sustainability of the project relies on retaining some of these new-comers. This research aims to identify the barriers that hinder newcomers' onboarding to OSS projects. Our method consisted of a qualitative study conducted with data obtained from four different sources: (i) systematic literature review, (ii) feedback from nine graduate and undergraduate students after they tried to join OSS projects, (iii) 24 responses to a questionnaire sent to 9 OSS projects, and (iv) semi-structured interviews with 36 sub-jects from 14 different projects, including newcomers and experi-enced members. The method to select the candidate papers in the systematic literature review was querying four digital libraries and backward snowballing. The data obtained from the practi-tioners from all three sources, and the primary studies obtained in the systematic review were analyzed using used procedures of Grounded Theory's open and axial coding. The analysis resulted in a conceptual model composed of 58 barriers, grouped into six different categories: cultural differences, newcomers' characteris-tics, reception issues, orientation, technical hurdles, and docu-mentation problems. We could observe recurrent barriers evi-denced in different data sources. We could notice that the onboarding process of a newcomer to an OSS can be a tough task. This research brings empirical support relying on data from different sources, organizes and discusses the existing common wisdom about barriers faced by newcomers to OSS projects, which deserve attention from researchers and OSS communities.