Download of Omni Outliner Professional 3.10.5 for Mac was on the developer's website when we last checked. We cannot confirm if there is a free download of this app available. Omnioutlinerpro-3.10.5.dmg is the frequent file name to indicate this application's installer.
Omni Outliner Professional for Mac belongs to Productivity Tools. The actual developer of this Mac application is The Omni Group. Please check the Mac app with an antivirus before launch as it is downloaded from the developer's website, and we cannot ensure that it is safe. The most popular version of the tool is 3.1.
From the developer:
OmniOutliner is a flexible program for creating, collecting, and organizing information. OmniOutliner's document structure can be used to brainstorm new ideas, drill out specifics, and line up the steps needed to get everything done. It's more than just an outlining tool—you've got multiple columns, smart checkboxes, customizable pop-up lists.
You may want to check out more Mac applications, such as OmniOutliner Professional, OmniOutliner or English <-> German Talking Dictionary Langenscheidt Professional, which might be similar to Omni Outliner Professional.