Context:
Open-source software (OSS) ecosystems are pivotal to modern software development by fostering innovation through inter-organizational collaboration. While becoming increasingly important, research on the patterns and dynamics of this collaboration in OSS remains limited.

Objective:
This study investigates inter-organizational collaboration within different OSS ecosystems, focusing on identifying key contributing and influential organizations, analyzing collaboration patterns, and understanding their evolution over time.

Method:
An exploratory case study with a mixed-methods approach was conducted, with data collected from five prominent OSS ecosystems on GitHub (React, Vue, TensorFlow, Bootstrap, and Flutter) involving 9947 developers and 339 organizations over ten years. Both qualitative and quantitative analyses were performed, including software repository mining and applying different levels (micro/macro) and dimensions (rational/structural/functional) of social network analysis (SNA) to examine collaboration patterns.

Results:
Key organizations, such as Alphabet, TensorFlow, Meta, Nvidia, Intel, MobileIron, IBM, and AMD, emerged as significant contributors, acting as central hubs for collaboration within OSS ecosystems. The collaboration networks showed an initial phase of rapid growth followed by stabilization, indicating project maturation. Notably, competing firms were found to contribute to the same ecosystems, underpinning the inherently cooperative, reciprocal, and multiplex nature of OSS development.

Conclusion:
The findings highlight the pivotal role of top-contributing organizations in fostering collaboration and driving innovation within OSS ecosystems. Understanding these dynamics provides valuable insights for developing strategies to enhance the effectiveness, scalability, and long-term sustainability of OSS ecosystems, benefiting both individual contributors and participating organizations.