The open-source office software world has erupted into a public feud, with The Document Foundation—the nonprofit behind LibreOffice—issuing a blistering statement accusing rival OnlyOffice of misrepresenting itself as open-source software while quietly aligning with Microsoft to perpetuate vendor lock-in. The allegations strike at the heart of what it means to be “open source” and raise pointed questions about the future of document interoperability in enterprise computing.
The controversy centers on OnlyOffice’s relationship with Microsoft’s proprietary OOXML document formats and what The Document Foundation characterizes as a deliberate strategy to keep users tethered to Microsoft’s file format standards rather than embracing the truly open Open Document Format (ODF). In a statement reported by Neowin, The Document Foundation pulled no punches, calling OnlyOffice’s open-source credentials into serious question and framing the dispute as a matter of user freedom versus corporate control.
A War of Words Over What ‘Open Source’ Really Means
The Document Foundation’s critique is multifaceted but boils down to a central argument: OnlyOffice, despite marketing itself as an open-source alternative to Microsoft Office, has built its product around Microsoft’s OOXML format as its native file format. This, The Document Foundation argues, effectively makes OnlyOffice an extension of the Microsoft ecosystem rather than a genuine alternative to it. By defaulting to OOXML—the format used by .docx, .xlsx, and .pptx files—OnlyOffice ensures that documents created in its software remain most compatible with Microsoft Office, reinforcing Microsoft’s dominance rather than challenging it.
LibreOffice, by contrast, uses ODF as its native format. ODF is an ISO-standardized, fully open document format that is not controlled by any single vendor. The Document Foundation has long championed ODF as the only format that guarantees long-term document accessibility and freedom from vendor lock-in. The foundation’s position is that any office suite claiming to be open source should prioritize open standards, and that defaulting to a format controlled by Microsoft fundamentally undermines the promise of open-source software.
The OOXML Question: Technical Standard or Corporate Trojan Horse?
The history of OOXML is itself a contentious chapter in the standards wars. Microsoft pushed OOXML through the ISO standardization process in 2008 amid widespread controversy, with critics alleging that the process was manipulated through irregular voting practices in several national standards bodies. Despite receiving ISO certification, OOXML has been criticized for being excessively complex—running to thousands of pages of specification—and for containing features that are effectively only implementable by Microsoft’s own software. The Document Foundation and other open-source advocates have long argued that OOXML’s ISO status provides a veneer of openness over what remains a proprietary-first format.
OnlyOffice has positioned its OOXML compatibility as a feature, not a bug. The company argues that because the vast majority of documents in circulation today are in Microsoft formats, offering native OOXML support provides the best user experience and the smoothest transition for organizations moving away from Microsoft Office. This is a pragmatic argument that resonates with many IT administrators who have struggled with formatting inconsistencies when moving documents between LibreOffice and Microsoft Office. But The Document Foundation contends that this pragmatism comes at a steep cost: it locks users into Microsoft’s format hegemony and ensures that Microsoft retains ultimate control over how documents are created, stored, and shared.
The ‘Fake Open Source’ Accusation Cuts Deep
Perhaps the most incendiary element of The Document Foundation’s statement is the characterization of OnlyOffice as “fake open source.” While OnlyOffice does publish much of its code under the GNU Affero General Public License (AGPL), The Document Foundation’s critique goes beyond licensing technicalities. The argument is that true open-source philosophy requires more than just publishing code—it requires a commitment to open standards, user freedom, and independence from proprietary vendor control. By building its product strategy around Microsoft’s format dominance, The Document Foundation suggests, OnlyOffice is open source in letter but not in spirit.
As reported by Neowin, The Document Foundation specifically pointed to OnlyOffice’s collaboration with Microsoft as evidence of this alignment. The foundation suggested that OnlyOffice’s development priorities are shaped by the need to maintain compatibility with Microsoft’s ecosystem rather than by the needs of the open-source community. This is a serious charge in the open-source world, where independence from corporate influence is considered a foundational value.
OnlyOffice’s Rise Has Made It a Real Competitor—and a Real Target
The timing of The Document Foundation’s broadside is notable. OnlyOffice has grown significantly in recent years, particularly in the European market where governments and public institutions have been actively seeking alternatives to Microsoft Office. The European Union’s push for digital sovereignty and open standards has created a fertile market for office suites that can claim open-source credentials while still handling the Microsoft-format documents that dominate enterprise workflows. OnlyOffice’s pitch—open source with excellent Microsoft format compatibility—has proven compelling to many procurement officers who want to check the open-source box without sacrificing day-to-day compatibility.
This growth has come partly at LibreOffice’s expense. While LibreOffice remains the most widely deployed open-source office suite, its formatting fidelity with Microsoft documents has been a persistent pain point. Complex Word documents with advanced formatting, Excel spreadsheets with intricate macros, and PowerPoint presentations with embedded media frequently lose formatting or functionality when opened in LibreOffice. OnlyOffice’s focus on OOXML compatibility has allowed it to claim superiority in this area, making it an attractive option for organizations that need to exchange documents regularly with Microsoft Office users.
The Broader Stakes: Document Sovereignty in the Age of Cloud Computing
The LibreOffice-OnlyOffice dispute reflects a larger tension in the technology industry between pragmatism and principle. As more organizations move their document workflows to cloud-based platforms, the question of which file formats serve as the default becomes increasingly consequential. Microsoft’s dominance in enterprise productivity—through Microsoft 365 and its associated cloud services—means that OOXML formats are deeply embedded in the workflows of millions of organizations worldwide. Any competitor that wants to win enterprise customers must reckon with this reality.
The Document Foundation’s position is that accommodating this reality is different from embracing it. LibreOffice supports OOXML formats for import and export but maintains ODF as its native format, a distinction the foundation considers essential. The argument is that by making ODF the default, LibreOffice gives users a genuine choice and ensures that their documents are stored in a format that no single company controls. OnlyOffice’s approach, by contrast, is characterized by the foundation as capitulation dressed up as pragmatism.
European Policy Could Tip the Balance
The dispute has particular relevance in Europe, where multiple governments have adopted or are considering policies that mandate or prefer open document formats for public sector use. Germany, France, and the Netherlands have all taken steps toward requiring ODF in government communications and document storage. The European Commission itself has promoted open standards as part of its digital strategy. In this regulatory environment, the distinction between an office suite that defaults to ODF and one that defaults to OOXML is not merely philosophical—it can determine which products qualify for government procurement contracts worth hundreds of millions of euros.
For OnlyOffice, the accusation of being “fake open source” could carry real commercial consequences in markets where open-source credentials are a procurement requirement. If government buyers accept The Document Foundation’s framing, OnlyOffice could find itself excluded from contracts it might otherwise have won. Conversely, if buyers prioritize practical Microsoft compatibility over format purity, OnlyOffice’s approach could continue to gain ground.
What This Means for Users Caught in the Middle
For the millions of individual users and thousands of organizations that rely on open-source office software, the dispute highlights an uncomfortable truth: choosing an office suite is not just a technical decision but a political one. The format your software defaults to shapes how your documents interact with the rest of the world, who controls the standards your data conforms to, and how accessible your files will be decades from now. LibreOffice’s insistence on ODF is rooted in a long-term vision of document freedom. OnlyOffice’s embrace of OOXML is rooted in the practical reality of a world where Microsoft formats dominate.
Neither position is without merit, and neither is without cost. Users who choose LibreOffice may face formatting headaches when exchanging documents with Microsoft Office users. Users who choose OnlyOffice may find themselves perpetuating the very vendor lock-in they sought to escape. The Document Foundation’s public attack on OnlyOffice has forced this trade-off into the open, ensuring that the debate over what “open source” truly means will continue to shape the office software market for years to come.
As the open-source community digests these charges and OnlyOffice formulates its response, one thing is clear: the battle for the future of document formats is far from settled, and the definition of “open” remains fiercely contested territory.