The fact that the number of users of open source software (OSS) is practically un-limited and that ultimately the software quality is determined by end users experience, makes the usability an even more critical quality attribute than it is for proprietary software. With the sharp increase in use of open source projects by both individuals and organizations, the level of usability and related issues must be addressed more seriously. The research model of this empirical investigation studies and establishes the relationship between the key usability factors from contributors perspective and OSS usability. A data set of 78 OSS contributors that includes architects, designers, developers, testers and users from 22 open source projects of varied size has been used to study the research model. The results of this study provide empirical evidence by indicating that the highlighted key factors play a significant role in improving OSS usability.