What is the OTF file format?
.otf files contain OpenType font definitions. The OpenType standard was jointly developed by Adobe and Microsoft, aiming to be the universal font standard supported by all software and hardware platforms. One of the means to achieve this goal is wide support for Unicode encoding and an extensive set of typographic functionalities, including:
- alternative characters (stylistic alternates, swashes and ornaments),
- support for advanced text-setting via
GSUB/GPOSlayout tables (ligatures, small caps, contextual substitution, precise kerning), - web delivery through WOFF2 wrappers when serving fonts directly from a website.
It is worth noting that fonts saved in .otf files are fully scalable, which means their size can be changed without loss of quality. .otf fonts are based on cubic Bézier curves - specifically PostScript/CFF outlines stored in the CFF table - which preserves quality without requiring conversion, as distinct from the quadratic curves used in .ttf files. .otf fonts always preserve their original design.
.otf is a multi-platform, multi-language standard especially useful when working with non-Latin alphabets such as Chinese or Arabic, supporting up to 65,535 glyphs per file. The compact CFF glyph data keeps file sizes lean compared with some older font formats. The standard also offers a function to digitally sign the font set via an optional DSIG table, to preserve the integrity and authenticity of .otf fonts.
Security & safety
RISK: LOWAn OTF is font data, not executable code, so it's safe to install in normal use. Two real cautions: font rasterizers (in the OS, browsers and PDF readers) have a long CVE history of being exploited by malformed fonts, so only install fonts from reputable sources and keep your system patched. And mind licensing/embedding: the OS/2 fsType flags say whether a font may be embedded or installed, and many 'free' fonts are free for personal use only - commercial use, web embedding, or app bundling may need a paid license. Avoid sketchy 'free font' sites whose downloaders bundle adware; prefer the foundry, Google Fonts, or Adobe Fonts.
Format details
in a nutshellPrograms that open OTF files
Windows3 apps
FontForge
Open-source Open the .otf to inspect/edit glyphs and metrics; File > Generate Fonts to export OTF/TTF/WOFF. 
FontCreator
Paid Open and edit the .otf (glyphs, kerning, OpenType features) in a polished Windows font editor.
Windows Font Viewer / Install (built-in)
Built-in Double-click the .otf to preview it, then click Install (or right-click > Install for all users). 
macOS2 apps
Font Book (built-in)
Built-in Double-click the .otf to preview, then click Install Font; manage and validate fonts in Font Book. 
Linux2 apps
Install to ~/.fonts or /usr/share/fonts
Built-in Copy the .otf to ~/.local/share/fonts (or /usr/share/fonts) and run fc-cache -f to register it.
GNOME Fonts / FontForge
Open-source Double-click in GNOME Fonts to preview/Install, or open in FontForge to edit.
Transfonter / online font tools
Free Upload an .otf to preview it and convert to WOFF/WOFF2 (or to TTF) client-side. Technical details
deep spec| MIME type | font/otf (also application/x-font-otf, application/vnd.ms-opentype) |
| File signature (magic bytes) | 'OTTO' (hex 4F 54 54 4F) at offset 0 - identifies CFF/PostScript-flavored OpenType |
| Byte order | Big-endian |
| Container format | sfnt - binary table directory with tagged, offset-addressed tables |
| Outline type | PostScript/CFF - cubic Bézier curves (Type 2 charstrings in the CFF table) |
| Core tables | CFF (outlines), cmap (char-to-glyph), head, hhea, hmtx, name, OS/2, post, GSUB, GPOS, GDEF |
| Compression | None in raw .otf; WOFF and WOFF2 wrappers add zlib/Brotli compression for web delivery |
| Maximum glyphs | 65,535 per font (16-bit glyph index) |
| Units per em (UPM) | 1,000 UPM is standard for CFF-flavored OpenType (vs. 2,048 for TrueType) |
| Integrity / checksums | Per-table checksums plus checkSumAdjustment in the head table; optional DSIG table for digital signing |
| Color font support | COLR/CPAL tables (layered vector), sbix (Apple bitmap), CBDT/CBLC (PNG bitmaps) - all optional extensions |
| Embedding permissions | fsType bit-field in the OS/2 table controls installable, editable, print/preview and no-embedding rights |
| Unicode coverage | Full Unicode including supplementary planes; glyph-to-character mapping defined in the cmap table |
| Typical file size | 30 KB - 5 MB for Latin families; large CJK OpenType fonts may reach tens of MB |
| Released | 1996 (announced by Microsoft and Adobe); first fonts shipped 2000-2001 |
| Latest version | OpenType 1.9.x; standardized as ISO/IEC 14496-22 'Open Font Format', current edition 2019 |
| Open standard | Yes · royalty-free |
| Specification | learn.microsoft.com |
OTF conversions
Community Q&A
asked by usersNo questions yet - be the first to ask about OTF files.
Frequently asked questions
How do I install an OTF font?
fc-cache -f. The font then appears in your apps.What's the difference between OTF and TTF?
How do I convert an OTF to WOFF2 for my website?
Why won't my OTF install or show up in my app?
fc-cache -f).
