The paper examines Open Source software adoption phenomenon by individual in an organizational context. It aims to identify factors that may be involved in the adoption process. To do this, a conceptual model was exhibited from previous IT adoption theories. Comparing to previous studies, it includes technical and organizational factors that influence the individual intention to adopt an OSS. To validate theoretical constructs, an exploratory qualitative study was conducted, in a first stage, to adapt the model to the Tunisian context specificities. Thus, an Open Source Software Adoption Model OSSAM is obtained. In a second stage, a quantitative confirmatory study was made to validate OSSAM. Data gathered by a survey (205 professionals) was analysed under a structural equation modeling approach (Partial Least Square). Research results provide important theoretical and practical contributions in the IT adoption area.