#_O_EXTENSIONS

.O    Object file (FreeBasic developer environments and editors).
.O    Object file (Many applications).
.O    Output file (Many applications).
.OB2  Oberon-2 source code file.
.OBD  Office Binder file (Microsoft Office).
.OBJ  WaveFront Object file.
.OBT  CM-Projekt file.
.OBT  Office Binder Template file (Microsoft Office).
.OBZ  Office Binder Wizard file (Microsoft Office).
.OCT  Octave script file (Michael Margraf's QUCS/Quite Universal Circuit Simulator).
.ODB  Open Document format Database file (Many applications).
.ODC  Microsoft Office Data Connection file (Microsoft Office XP Professional).
.ODF  Hybrid PDF/ODF document = Standard PDF file containing an ODF document (OpenOffice-based applications with PDF support).
.ODF  Open Document format Formula file (Many applications).
.ODG  Open Document format Drawing (Many applications, OpenOffice, Libre Office, etc.).
.ODG  Open Document format Graphics vector image (OpenOffice, Libre Office, etc.).
.ODM  Open Document format Master document (Office applications).
.ODP  Open Document format Presentation template file (Apache OpenOffice, Calligra Suite, LibreOffice, NeoOffice, and other Office suites).
.ODS  Open Document format Spreadsheet file (Apache OpenOffice, Calligra Suite, LibreOffice, NeoOffice, and other Office suites).
.ODT  Open Document format Text file (Many Office suites/applications).
.OFL  Open Font License file = Text.
.OFR  OptimFROG format audio file = Florin Ghido's OptimFROG is a lossless audio codec.
.OFS  OptimFROG format audio file = Florin Ghido's OptimFROG is a lossless audio codec.
.OFT  Outlook e-mail message File Template file (Microsoft Office XP Outlook).
.OFX  OpenFX Animator format file (OpenFX).
.OGA  Ogg Vorbis audio Album file (Many audio applications).
.OGG  Ogg Vorbis audio file (Many audio applications).
.OGV  Ogg Vorbis Video file (Many audio applications).
.OMF  Open Media Framework video file (Adobe Systems' Premiere, Adobe Systems' Audition).
.OMS  Open Music System file (Many audio/MIDI applications).
.ONE  Onespace page file (Enfish Onespace Personal).
.ONS  High score file (Lasse Rautiainen's Golf).
.OPML Open Markup Language file = XML based markup language (Many applications).
.OPS  Office Profile Settings file (Microsoft Office Profile Settings Wizard).
.OPZ  Outback Plus mail archive = OutLook mail backup (AJ Systems' Outback Plus).
.OQY  OLAP Query file (Microsoft Excel).
.OR2 .OR5 Lotus Organizer files. OR2=v2 OR3=v97 OR4=4.1 and v97GS OR5=v5 (Lotus Development's Lotus Organizer).
.ORA  OpenRaster Archiving image file (Martin Renold's MyPaint and many other graphic image applications).
.ORC  CSound orchestra file (Algorithmic composition applications that employ the CSoundAC API, CSoundQt).
.ORF  Olympus RAW Format image file (Olympus cameras, many image applications, many variants).
.ORG  Lotus Organizer files (Lotus Development's Lotus Organizer 1).
.ORK  OpenRocket design project file (Sampo Niskanen's OpenRocket rocket simulator).
.ORP  Orangator Presets file (Agent Orange and Tangerine's Orangator).
.OTF  OpenType Font files (Many applications).
.OTH  HTML document template.
.OTL  Outline file (Fookes Holdings' NoteTab and other text editors).
.OTO  Panorama stitching task data file (Hugin PanoTools, AutoPano, and other panorama/stitching software packages).
.OTT  Open Document format Text Template file (Calligra Words and many other word processing applications).
.OUT  Bundler Output file (Visual Computing Lab - ISTI - CNR's MeshLab).
.OUT  Output placeholder file = An empty temporary file which is renamed to a file or folder before content is added/extracted into it (Smith Micro's Stuffit Deluxe and Stuffit Expander).
.OUTLINE Letter Outline file (Neuber Font Twister).
.OVL  Overlay file (Many applications and variants).
.OXT  Open Extension file (LibreOffice, OpenOffice and other Office applications/suites).
.OZ   Bitmap character table graphics file (Oz & Kenichi's Tower of the Sorcerer).