Many commercial software firms rely on opensourcing as a viable model of software production. Opensourcing is a specific form of interaction between firms and open source software (OSS) communities for collaboratively producing software. The existing literature has identified opensourcing as a viable form of software production, which could be a substitute for “in-house” or “outsourced” software development. However, little is known about how opensourcing works or is sustained in the long term. The objective of this research is to explain the factors affecting the sustainability of opensourcing as a model of software production. The study employs a single case study of hospital software in Thailand to understand how firms and the communities can live symbiotically and sustain their collaboration to peer-produce vertical domain software. The analysis reveals six mechanisms (positive experience, trust in the leadership of the project leader, the demonstration of reciprocity, marketing the community, enriching knowledge, and face-to-face meetings) and demonstrates how they operate in conjunction with each other to sustain opensourcing.