International Symposium on Software Composition (OOo), commonly known as OpenOffice, is a discontinued open-source office suite. It was an open-sourced version of the earlier StarOffice, which Sun Microsystems acquired in 1999 for internal use.

Apache renamed the software Apache OpenOffice. Other active successor projects include LibreOffice (the most actively developed) and NeoOffice (commercial, only for macOS).

On 19 July 2000 at OSCON, Sun Microsystems announced it would make the source code of StarOffice available for download with the intention of building an open-source development community around the software and of providing a free and open alternative to Microsoft Office. The new project was known as, and the code was released as open source on 13 October 2000. The first public preview release was Milestone Build 638c, released in October 2001 (which quickly achieved 1 million downloads); the final release of 1.0 was on 1 May 2002.

Development of was sponsored primarily by Sun Microsystems, which used the code as the basis for subsequent versions of StarOffice. Developers who wished to contribute code were required to sign a Contributor Agreement granting joint ownership of any contributions to Sun (and then Oracle), in support of the StarOffice business model. This was controversial for many years. An alternative Public Documentation Licence (PDL) was also offered for documentation not intended for inclusion or integration into the project code base.

In June 2011, Oracle contributed the trademarks to the Apache Software Foundation. It also contributed Oracle-owned code to Apache for relicensing under the Apache License, at the suggestion of IBM (to whom Oracle had contractual obligations concerning the code), as IBM did not want the code put under a copyleft license. This code drop formed the basis for the Apache OpenOffice project.

Due to a similar trademark issue (a Rio de Janeiro company that owned that trademark in Brazil), the Brazilian Portuguese version of the suite was distributed under the name from 2004, with BrOffice.Org being the name of the associated local nonprofit from 2006. ( moved to LibreOffice in December 2010.)

Fontwork is a feature that allows users to create stylized text with special effects differing from ordinary text with the added features of gradient colour fills, shaping, letter height, and character spacing. It is similar to WordArt used by Microsoft Word. When saved documents in Microsoft Office file format, all Fontwork was converted into WordArt.

Although originally written in C++, became increasingly reliant on the Java Runtime Environment, even including a bundled JVM. was criticized by the Free Software Foundation for its increasing dependency on Java, which was not free software.

The preview, Milestone 638c, was released October 2001. 1.0 was released under both the LGPL and the SISSL for Windows, Linux and Solaris on 1 May 2002. The version for Mac OS X (with X11 interface) was released on 23 June 2003.

Work on version 2.0 began in early 2003 with the following goals (the "Q Product Concept"): better interoperability with Microsoft Office; improved speed and lower memory usage; greater scripting capabilities; better integration, particularly with GNOME; a more usable database; digital signatures; and improved usability. It would also be the first version to default to OpenDocument. Sun released the first beta version on 4 March 2005.

Version 3.2 included support for PostScript-based OpenType fonts. It warned users when ODF 1.2 Extended features had been used. An improvement to the document integrity check determined if an ODF document conformed to the ODF specification and offered a repair if necessary. Calc and Writer both reduced "cold start" time by 46% compared to version 3.0. 3.2.1 was the first Oracle release.