Developing systems by integrating Open Source Software (OSS) is increasingly gaining importance in the software industry. Although the literature claims that this approach highly impacts Requirements Engineering (RE) practices, there is a lack of empirical evidence to demonstrate this statement. To explore and understand problems and challenges of current system requirement–OSS component matching and mismatches resolution practices in software development projects that integrate one or more OSS components into their software products. Semi-structured in-depth interviews with 25 respondents that have performed RE activities in software development projects that integrate OSS components in 25 different software development companies in Spain, Norway, Sweden, and Denmark. The study uncovers 15 observations regarding system requirements-OSS components matching and mismatch resolution practices used in industrial projects that integrate OSS components. The assessed projects focused mainly on pre-release stages of software applications that integrate OSS components in an opportunistic way. The results also provide details of a set of previously unexplored scenarios when solving system requirement–OSS component mismatches; and clarify some challenges and related problems. For instance, although licensing issues and the potential changes in OSS components by their corresponding communities and/or changes in system requirements have been greatly discussed in the RE literature as problems for OSS component integration, they did not appear to be relevant in our assessed projects. Instead, practitioners highlighted the problem of getting suitable OSS component documentation/information.