O

open source
Open-source software is software that may be used and redistributed without paying a licensing fee, and includes a copy of its source code. Vendors may sell distributions that include both the open-source software and their own added-value components (including technical support). Open-source software is popular with large organizations that often need to customize an application or OS.

operating system (OS)
An OS is the basic system software of a computer that provides low-level services to applications.