Files with the .oo3 extension are OmniOutliner 3 Format files defined by The Omni Group. These data files may be used by programs for the Mac OS platform. The most popular software supporting .oo3 files is Omni Group Outliner. Other programs can also handle .oo3 files, yet there is no guarantee each will properly support them on all platforms including mobile.
To open a .oo3 file, double-click on its icon. If your system has the appropriate application associated with .oo3 files, the file should open. If not, installing the Omni Group Outliner may solve this issue since applications often create file associations during installation.
Below is a list of applications supporting .oo3 files:
Omni Group Outliner
A file extension is the 3-4 character code at the end of a filename indicating the file type. Windows uses extensions to associate default programs with each file type. When the associated program is no longer on your PC, errors can occur when trying to open the file.
You can fix .oo3 file association errors by finding and repairing file extension issues, registry problems and restoring PC performance using utilities like Registry Reviver. This will allow you to open .oo3 files correctly.
To always open .oo3 files with a specific program, right click on any .oo3 file and choose “Open with”>”Choose another app”. Select the desired program and check “Always use this app to open .oo3 files”.
OmniOutliner for Mac